MEMBRANE ELECTRODE ASSEMBLY FOR COx REDUCTION

ABSTRACT

Provided herein are membrane electrode assemblies (MEAs) for COx reduction and carbon dioxide reduction reactors (CRRs) that include MEAs.

INCORPORATION BY REFERENCE

An Application Data Sheet is filed concurrently with this specificationas part of the present application. Each application that the presentapplication claims benefit of or priority to as identified in theconcurrently filed Application Data Sheet is incorporated by referenceherein in its entirety and for all purposes.

SUMMARY

Provided herein are membrane electrode assemblies (MEAs) for CO_(x)reduction. According to various embodiments, the MEAs are configured toaddress challenges particular to CO_(x) including managing water in theMEA. Bipolar and anion-exchange membrane (AEM)-only MEAs are described.

One aspect of the disclosure relates to a membrane electrode assemblyincluding a cathode catalyst layer; an anode catalyst layer; and abipolar membrane disposed between the cathode catalyst layer and theanode catalyst layer, wherein the bipolar membrane includes ananion-conducting polymer layer, a cation-conducting polymer layer, and abipolar interface between the anion-conducting polymer layer and thecation-conducting polymer layer, wherein the cation-conducting polymerlayer is disposed between the anode catalyst layer and theanion-conducting polymer layer, and wherein the bipolar interface ischaracterized by or includes one or more of:

-   -   covalent cross-linking of the cation-conducting polymer layer        with the anion-conducting polymer layer;    -   interpenetration of the anion-conducting polymer layer and the        cation-conducting polymer layer; and    -   a layer of a second anion-conducting polymer, wherein the ion        exchange capacity of the second anion-conducting polymer is        higher than the ion exchange capacity of the anion-conducting        polymer of the anion-conducting polymer layer.

In some embodiments, the bipolar interface is characterized byinterpenetration of the anion-conducting polymer layer and thecation-conducting polymer layer and the region of interpenetration isbetween 10% and 75% of the total thickness of the anion-conducting layerincluding the interpenetration region. In some embodiments, the bipolarinterface includes protrusions having a dimension of between 10 μm-1 mmin a plane parallel to the anion-conducting polymer layer (the in-planedimension). In some embodiments, the bipolar interface is characterizedby interpenetration of the anion-conducting polymer layer and thecation-conducting polymer layer and wherein the bipolar interfaceincludes protrusions each having a thickness of between 10% to 75% ofthe total thickness of the anion-conducting polymer layer. In someembodiments, the bipolar interface is characterized by interpenetrationof the anion-conducting polymer layer and the cation-conducting polymerlayer and wherein the bipolar interface includes a gradient of theanion-conducting polymer and/or the cation-conducting polymer. In someembodiments, the bipolar interface is characterized by interpenetrationof the anion-conducting polymer layer and the cation-conducting polymerlayer and wherein the bipolar interface includes a mixture of theanion-conducting polymer and/or the cation-conducting polymer.

In some embodiments, the bipolar interface includes a layer of a secondanion-conducting polymer, and further wherein the thickness of the layerof the second anion-conducting polymer is between 0.1% and 10% of thethickness of the anion-conducting polymer layer. In some embodiments,the bipolar interface includes a layer of a second anion-conductingpolymer and further wherein the second anion-conducting polymer has anion exchange capacity (IEC) of between 2.5 and 3.0 mmol/g. ISSE, theanion-conducting polymer has an IEC of between 1.5 and 2.5 mmol/g. Insome embodiments, the bipolar interface includes a layer of a secondanion-conducting polymer and wherein the second anion-conducting polymerhas a lower water uptake than that of the anion-conducting polymer ofthe anion-conducting polymer layer.

In some embodiments, bipolar interface includes covalent crosslinking ofthe cation-conducting polymer layer with the anion-conducting polymerlayer and the covalent crosslinking includes a material including astructure of one of formulas (I)-(V), (X)-(XXXIV) as described furtherbelow, or a salt thereof.

In some embodiments, the bipolar interface includes covalentcrosslinking of the cation-conducting polymer layer with theanion-conducting polymer layer and wherein the covalent crosslinkingincludes a material including a structure of one of formulas (I)-(V):

or a salt thereof,

-   -   wherein:    -   each of R⁷, R⁸, R⁹, and R¹⁰ is, independently, an        electron-withdrawing moiety, H, optionally substituted        aliphatic, alkyl, heteroaliphatic, heteroalkylene, aromatic,        aryl, or arylalkylene, wherein at least one of R⁷ or R⁸ can        include the electron-withdrawing moiety or wherein a combination        of R⁷ and R⁸ or R⁹ and R¹⁰ can be taken together to form an        optionally substituted cyclic group;    -   Ar includes or is an optionally substituted aromatic or arylene;    -   each of n is, independently, an integer of 1 or more;    -   each of rings a-c can be optionally substituted; and    -   rings a-c, R⁷, R⁸, R⁹, and R¹⁰ can optionally include an        ionizable or ionic moiety.    -   ISSE, R⁷ or R⁸ includes the electron-withdrawing moiety selected        from the group consisting of an optionally substituted        haloalkyl, cyano, phosphate, sulfate, sulfonic acid, sulfonyl,        difluoroboranyl, borono, thiocyanato, and piperidinium.

In some embodiments, the bipolar interface includes covalentcrosslinking of the cation-conducting polymer layer with theanion-conducting polymer layer and wherein the covalent crosslinkingincludes a material including a structure of one of the followingformulas:

or a salt thereof, wherein:

-   -   Ar is or includes an optionally substituted arylene or aromatic;    -   Ak is or includes an optionally substituted alkylene,        haloalkylene, aliphatic, heteroalkylene, or heteroaliphatic; and    -   L is a linking moiety, and    -   wherein one or Ar, Ak, and/or L is optionally substituted with        one or more ionizable or ionic moieties.

In some embodiments, the bipolar interface includes covalentcrosslinking of the cation-conducting polymer layer with theanion-conducting polymer layer and wherein the covalent crosslinkingincludes a crosslinker including a structure of one of the followingformulas:

wherein:

-   -   Ak is an optionally substituted aliphatic or an optionally        substituted alkylene;    -   Ar is an optionally substituted aromatic or an optionally        substituted arylene;    -   L is a linking moiety;    -   L3 is an integer that is 2 or more; and    -   X′ is absent, —O—, —NR^(N1)—, —C(O)—, or -Ak-, in which R^(N1)        is H or optionally substituted alkyl, and Ak is optionally        substituted alkylene, optionally substituted heteroalkylene,        optionally substituted aliphatic, or optionally substituted        heteroaliphatic.

In some embodiments, the covalent crosslinking includes a materialincluding one or more ionizable or ionic moieties selected from thegroup consisting of -L^(A)-X^(A), -L^(A)-(L^(A′)-X^(A))_(L2),-L^(A)-(X^(A)-L^(A′)-X^(A′))_(L2), and-L^(A)-X^(A)-L^(A′)-X^(A′)-L^(A″)-X^(A″); wherein:

-   -   each L^(A), L^(A′), and L^(A″) is, independently, a linking        moiety;    -   each X^(A), X^(A′), and X^(A″) includes, independently, an        acidic moiety, a basic moiety, a multi-ionic moiety, a cationic        moiety, or an anionic moiety; and    -   L2 is an integer of 1 or more.

In some such embodiments, each X^(A), X^(A′), and X^(A″) includes,independently, carboxy, carboxylate anion, guanidinium cation, sulfo,sulfonate anion, sulfonium cation, sulfate, sulfate anion, phosphono,phosphonate anion, phosphate, phosphate anion, phosphonium cation,phosphazenium cation, amino, ammonium cation, heterocyclic cation, or asalt form thereof.

In some embodiments, the linking moiety includes a covalent bond,spirocyclic bond, —O—, —NR^(N1)—, —C(O)—, —C(O)O—, —OC(O)—, —SO₂—,optionally substituted aliphatic, alkylene, alkyleneoxy, haloalkylene,hydroxyalkylene, heteroaliphatic, heteroalkylene, aromatic, arylene,aryleneoxy, heteroaromatic, heterocycle, or heterocyclyldiyl.

Another aspect of the disclosure relates to a membrane electrodeassembly (MEA) including: a cathode layer; an anode layer; and a bipolarmembrane disposed between the cathode layer and the anode layer, whereinthe bipolar membrane includes a cation-conducting polymer layer and ananion-conducting polymer layer, wherein the cation-conducting polymerlayer is disposed between the anode layer and the anion-conductingpolymer layer, and wherein the thickness of the anion-conducting polymerlayer is between 5 and 80 micrometers.

In some embodiments, thickness of the anion-conducting polymer layer isbetween 5 and 50 micrometers. In some embodiments, the thickness of theanion-conducting polymer layer is between 5 and 40 micrometers. In someembodiments, the thickness of the anion-conducting polymer layer isbetween 5 and 30 micrometers.

In some embodiments, the molecular weight of the anion-conductingpolymer is at least 30 kg/mol, at least 45 kg/mol, or at least 60kg/mol.

In some embodiments, wherein the ratio of the thickness of thecation-conducting polymer layer to the thickness anion-conductingpolymer layer is at least 3:1. In some embodiments, the ratio of thethickness of the cation-conducting polymer layer to the thickness of theanion-conducting polymer layer is at least 7:1. In some embodiments, theratio of the thickness of the cation-conducting polymer layer to theanion-conducting polymer layer is at least 13:1.

In some embodiments, the ratio of the thickness of the cation-conductingpolymer layer to the thickness of the anion-conducting polymer layer isno more than 3:1. In some embodiments, the ratio of the thickness of thecation-conducting polymer layer to the thickness anion-conductingpolymer layer is no more than 2:1. In some embodiments, the ratio of thethickness of the cation-conducting polymer layer to the thickness of theanion-conducting polymer layer is no more than 1:1.

Another aspect of the disclosure relates to a membrane electrodeassembly including a cathode catalyst layer; an anode catalyst layer;and a bipolar membrane disposed between the cathode catalyst layer andthe anode catalyst layer, wherein the bipolar membrane includes ananion-conducting polymer layer, a cation-conducting polymer layer, and abipolar interface between the anion-conducting polymer layer and thecation-conducting polymer layer, wherein the cation-conducting polymerlayer is disposed between the anode catalyst layer and theanion-conducting polymer layer, and wherein the bipolar interface ischaracterized by or includes one or more of:

-   -   a material selected from an ionic liquid, a non-ionically        conductive polymer; a metal, an oxide ion donor, a catalyst; a        CO₂ absorbing material, and a H₂ absorbing material; and    -   a material that extends across and mechanically reinforces the        interface.

Another aspect of the disclosure relates to a membrane electrodeassembly (MEA) including: a cathode layer; an anode layer; and a bipolarmembrane disposed between the cathode layer and the anode layer, whereinthe bipolar membrane includes a cation-conducting polymer layer and ananion-conducting polymer layer, wherein the cation-conducting polymerlayer is disposed between the anode layer and the anion-conductingpolymer layer, and wherein the molecular weight of the anion-conductingpolymer is at least 30 kg/mol. In some embodiments, it is at least 45kg/mol or at least 60 kg/mol.

Another aspect of the disclosure relates to a membrane electrodeassembly (MEA) including: a cathode layer comprising reduction catalystand a first ion-conducting polymer, an anode layer comprising oxidationcatalyst and a second ion-conducting polymer, and a polymer electrolytemembrane comprising a third ion-conducting polymer between the anodelayer and the cathode layer. The polymer electrolyte membrane providesionic communication between the anode layer and the cathode layer. Insome embodiments, a cathode buffer layer comprising a fourthion-conducting polymer is between the cathode layer and the polymerelectrolyte membrane. At least two of the first, second, third, andfourth ion-conducting polymers are from different classes(anion-conductors, cation-conductors, and cation-and-anion-conductors)of ion-conducting polymers.

Another aspect of the disclosure relates to a CO_(x) reduction reactor.The reactor has at least one electrochemical cell, which comprises anyof the membrane electrode assemblies described herein. The reactor alsohas a cathode support structure adjacent to the cathode, the cathodesupport structure comprising a cathode polar plate, at least one cathodegas diffusion layer, at least one inlet and at least one outlet. Thereis also an anode cell support structure adjacent to the anode. The anodesupport structure comprises an anode polar plate and at least one anodegas diffusion layer, at least one inlet and at least one outlet.

In yet another aspect of the disclosure, a method of operating a CO_(x)reduction reactor is provided. The method results in production ofreaction products. The process can include: providing an electrochemicalreactor comprising at least one electrochemical cell comprising amembrane electrode assembly, a cathode support structure adjacent to thecathode that includes a cathode polar plate, at least one cathode gasdiffusion layer, at least one gas inlet and at least one gas outlet, andan anode cell support structure adjacent to the anode that includes ananode polar plate and at least one anode gas diffusion layer, at leastone inlet and at least one outlet; applying a DC voltage to the cathodepolar plate and the anode polar plate; supplying one or more oxidationreactants to the anode and allowing oxidation reactions to occur;supplying one or more reduction reactants to the cathode and allowingreduction reactions to occur; collecting oxidation reaction productsfrom the anode; and collecting reduction reaction products from thecathode.

Also provided are methods of fabrication of MEAs and anion-exchangemembrane (AEM)-only MEAs. These and other aspects of the disclosure arediscussed further below with reference to the drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shins a membrane electrode assembly used in a water electrolysisreactor, which makes hydrogen and oxygen.

FIGS. 2A-2C are schematic illustrations of a membrane electrodeassemblies (MEAs) for use in a CO_(x) reduction reactor (CRR) accordingto various embodiments.

FIG. 3 is a schematic illustration of a carbon dioxide (CO₂)electrolyzer configured to receive water and CO₂ (e.g., humidified ordry gaseous CO₂) as a reactant at a cathode and expel carbon monoxide(CO) as a product.

FIG. 4 illustrates an example construction of a CO_(x) reduction MEAhaving a cathode catalyst layer, an anode catalyst layer, and ananion-conducting polymer electrolyte membrane (PEM).

FIG. 5 illustrates an example construction of a CO reduction MEA 5having a cathode catalyst layer, an anode catalyst layer, and ananion-conducting PEM.

FIG. 6 is a schematic drawing that shows a possible morphology for twodifferent kinds of catalysts supported on a catalyst support particle.

FIG. 7 shows examples of shapes and sizes of metal catalyst particles.

FIG. 8 shows an example of a method according to certain embodiments inwhich an ionomer is used as a ligand to direct the synthesis of ananocrystal catalyst.

FIG. 9 is a schematic illustration of a bipolar interface of a MEA.

FIGS. 10A-10D are schematic illustrations of bipolar interfaces of MEAsthat are configured to resist delamination.

FIG. 11 is a schematic illustration of layers of a MEA, including ananion-conducting polymer layer (AEM) and a polymer electrolyte membrane(PEM).

FIG. 12 shows Faraday efficiency for COx electrolyzers having bipolarMEAs with different thicknesses of AEM.

FIG. 13 shows cell voltages for COx electrolyzers having bipolar MEAswith different thicknesses of AEM.

FIG. 14 is a schematic drawing that shows the major components of aCO_(x) reduction reactor (CRR) according to certain embodiments.

FIG. 15 is a schematic drawing that shows the major components of a CRRwith arrows showing the flow of molecules, ions, and electrons accordingto certain embodiments.

FIG. 16 is a schematic drawing that shows the major inputs and outputsof the CRR reactor.

DETAILED DESCRIPTION Introduction

Embodiments are illustrated in the context of reduction of CO_(x) (CO₂,CO, or combinations thereof) to produce useful chemicals and fuels. Theskilled artisan will readily appreciate, however, that the materials andmethods disclosed herein will have application in a number of othercontexts where reduction reactions are desirable, particularly whereproduction of a variety of chemicals in a variety of reaction conditionsis important. The reactor used to reduce CO_(x) could also be used toreduce other compounds, including but not limited to: N₂, SO_(x),NO_(x), acetic acid, ethylene, O₂ and any other suitable reduciblecompound or combinations thereof.

The below table lists some abbreviations that are used throughout thisapplication.

Abbreviation Description CO_(x) CO₂, CO or a combination thereof CRRCO_(x) reduction reactor MEA membrane electrode assembly PEM polymerelectrolyte membrane

Hydration is useful in effecting ion conduction for most ion-conductingpolymers. Humidification of COx or anode feed material can be used forthe delivery of liquid water to the MEA to maintain hydration ofion-conducting polymers.

In one embodiment of the invention, a COx reduction reactor (CRR) thatuses a novel membrane electrode assembly in an electrochemical cell hasbeen developed. The below table lists some examples of useful chemicalsthat can be produced from COx in such a reactor.

Example CO₂ and CO Reduction Products

Formic Acid Carbon Monoxide Methanol Glyoxal Methane Acetic AcidGlycolaldehyde Ethylene Glycol Acetaldehyde Ethanol EthyleneHydroxyacetone Acetone Allyl Alcohol Propionaldehyde n-Propanol Syngas

Membrane Electrode Assembly

One aspect of the disclosure is a membrane electrode assembly (MEA) isdescribed here. It may be used in a CO_(x) reduction reactor. CO_(x) maybe carbon dioxide (CO₂), carbon monoxide (CO), CO₃ ²⁻ (carbonate ion),HCO₃ ⁻ (bicarbonate ion), or combinations thereof. The MEA contains ananode layer, a cathode layer, electrolyte, and optionally one or moreother layers. The layers may be solids and/or soft materials. The layersmay include polymers such as ion-conducting polymers.

When in use, the cathode of an MEA promotes electrochemical reduction ofCO_(x) by combining three inputs: CO_(x), ions (e.g., protons) thatchemically react with CO_(x), and electrons. The reduction reaction mayproduce CO, hydrocarbons, and/or oxygen and hydrogen containing organiccompounds such as methanol, ethanol, and acetic acid. When in use, theanode of an MEA promotes an electrochemical oxidation reaction such aselectrolysis of water to produce elemental oxygen and protons. Thecathode and anode may each contain catalysts to facilitate theirrespective reactions.

The compositions and arrangements of layers in the MEA may promote highyield of a CO_(x) reduction products. To this end, the MEA mayfacilitate any one or more of the following conditions: (a) minimalparasitic reduction reactions (non-CO_(x) reduction reactions) at thecathode; (b) low loss of CO_(x) reactants at anode or elsewhere in theMEA; (c) maintain physical integrity of the MEA during the reaction(e.g., prevent delamination of the MEA layers); (d) prevent CO_(x)reduction product cross-over; (e) prevent oxidation production (e.g.,02) cross-over; (f) maintain a suitable environment at the cathode/anodefor oxidation/reduction as appropriate; (g) provide pathway for desiredions to travel between cathode and anode while blocking undesired ions;and (h) minimize voltage losses.

Cox Reduction Specific Problems

Polymer-based membrane assemblies such as MEAs have been used in variouselectrolytic systems such as water electrolyzers and in various galvanicsystems such as fuel cells. However, CO_(x) reduction presents problemsnot encountered, or encountered to a lesser extent, in waterelectrolyzers and fuel cells.

For example, for many applications, an MEA for CO_(x) reduction requiresa lifetime on the order of about 50,000 hours or longer (approximatelyfive years of continuous operation), which is significantly longer thanthe expected lifespan of a fuel cell for automotive applications; e.g.,on the order of 5,000 hours. And for various applications, an MEA forCO_(x) reduction employs electrodes having a relatively large geometricsurface area by comparison to MEAs used for fuel cells in automotiveapplications. For example, MEAs for CO_(x) reduction may employelectrodes having geometric surface areas (without considering pores andother nonplanar features) of at least about 500 cm². CO_(x) reductionreactions may be implemented in operating environments that facilitatemass transport of particular reactant and product species, as well as tosuppress parasitic reactions. Fuel cell and water electrolyzer MEASoften cannot produce such operating environments. For example, such MEASmay promote undesirable parasitic reactions such as gaseous hydrogenevolution at the cathode and/or gaseous CO₂ production at the anode.

In some systems, the rate of a CO_(x) reduction reaction is limited bythe availability of gaseous CO_(x) reactant at the cathode. By contrast,the rate of water electrolysis is not significantly limited by theavailability of reactant: liquid water tends to be easily accessible tothe cathode and anode, and electrolyzers can operate close to highestcurrent density possible.

MEA Configurations MEA General Arrangement

In certain embodiments, an MEA has a cathode layer, an anode layer, anda polymer electrolyte membrane (PEM) between the anode layer and thecathode layer. The polymer electrolyte membrane provides ioniccommunication between the anode layer and the cathode layer, whilepreventing electronic communication, which would produce a shortcircuit. The cathode layer includes a reduction catalyst and a firstion-conducting polymer. The cathode layer may also include an ionconductor and/or an electron conductor. The anode layer includes anoxidation catalyst and a second ion-conducting polymer. The anode layermay also include an ion conductor and/or an electron conductor. The PEMincludes a third ion-conducting polymer.

In certain embodiments, the MEA has a cathode buffer layer between thecathode layer and the polymer electrolyte membrane. The cathode bufferincludes a fourth ion-conducting polymer.

In certain embodiments, the MEA has an anode buffer layer between theanode layer and the polymer electrolyte membrane. The anode bufferincludes a fifth ion-conducting polymer.

In connection with certain MEA designs, there are three availableclasses of ion-conducting polymers: anion-conductors, cation-conductors,and mixed cation-and-anion-conductors. In certain embodiments, at leasttwo of the first, second, third, fourth, and fifth ion-conductingpolymers are from different classes of ion-conducting polymers.

For context, as shown in FIG. 1 , a membrane electrode assembly (MEA)100 used for water electrolysis has a cathode 120 and an anode 140separated by an ion-conducting polymer layer 160 that provides a pathfor ions to travel between the cathode 120 and the anode 140. Thecathode 120 and the anode 140 each contain ion-conducting polymer andcatalyst particles. One or both may also include electronicallyconductive catalyst support. The ion-conducting polymer in the cathode120, anode 140, and ion-conducting polymer layer 160 are either allcation-conductors or all anion-conductors.

The MEA 100 is not suitable for use in a carbon oxide reduction reactor(CRR). When all of the ion-conducting polymers are cation-conductors,the environment favors H₂ generation, an unwanted side reaction, at thecathode layer. The production of hydrogen lowers the rate of CO_(x)product production and lowers the overall efficiency of the process.

When all of the ion-conducting polymers are anion-conductors, then CO₂reacts with hydroxide anions in the ion-conducting polymer at thecathode to form bicarbonate anions. The electric field in the reactormoves the bicarbonate anions from the cathode side of the cell to theanode side of the cell. At the anode, bicarbonate anions can decomposeback into CO₂ and hydroxide. This results in the net movement of CO₂from the cathode to the anode of the cell, where it does not react andis diluted by the anode reactants and products. This loss of CO₂ to theanode side of the cell reduces the efficiency of the process.

Conductivity and Selectivity of Ion-Conducting Polymers for MEA Layers

The term “ion-conducting polymer” is used herein to describe a polymerelectrolyte having greater than about 1 mS/cm specific conductivity foranions and/or cations. The term “anion-conductor” describes anion-conducting polymer that conducts anions primarily (although therewill still be some small amount of cation conduction) and has atransference number for anions greater than about 0.85 at around 100micron thickness. The terms “cation-conductor” and/or “cation-conductingpolymer” describe an ion-conducting polymer that conducts cationsprimarily (e.g., there can still be an incidental amount of anionconduction) and has a transference number for cations greater thanapproximately 0.85 at around 100 micron thickness. For an ion-conductingpolymer that is described as conducting both anions and cations (a“cation-and-anion-conductor”), neither the anions nor the cations has atransference number greater than approximately 0.85 or less thanapproximately 0.15 at around 100 micron thickness. To say a materialconducts ions (anions and/or cations) is to say that the material is anion-conducting material or ionomer. Examples of ion-conducting polymersof each class are provided in the below Table.

Ion-Conducting Polymers Class Description Common Features Examples A.Anion- Greater than Positively charged aminated tetramethyl conductingapproximately 1 mS/cm functional groups are polyphenylene, specificconductivity covalently bound to poly(ethylene-co- for anions, whichhave the polymer tetrafluoroethylene)-based a transference numberbackbone quaternary ammonium greater than polymer, quaternizedapproximately 0.85 at polysulfone around 100 micron thickness B.Conducts Greater than Salt is soluble in the polyethylene oxide; bothanions and approximately 1 mS/cm polymer and the salt polyethyleneglycol; cations conductivity for ions ions can move poly(vinylidenefluoride); (including both cations through the polymer polyurethane andanions), which have material a transference number between approximately0.15 and 0.85 at around 100 micron thickness C. Cation- Greater thanNegatively charged perfluorosulfonic acid conducting approximately 1mS/cm functional groups are polytetrafluoroethylene specificconductivity covalently bound to co-polymer; sulfonated for cations,which have the polymer poly(ether ether ketone); a transference numberbackbone poly(styrene sulfonic acid- greater than co-maleic acid)approximately 0.85 at around 100 micron thickness

Some Class A ion-conducting polymers are known by tradenames such as2259-60 (Pall RAI), AHA by Tokuyama Co, Fumasep® FAA-(fumatech GbbH),Sustanion®, Morgane ADP by Solvay, or Tosflex® SF-17 by Tosoh anionexchange membrane material. Further class A ion-conducting polymersinclude HNN5/HNN8 by Ionomr, FumaSep by Fumatech, TM1 by Orion, andPAP-TP by W7energy. Some Class C ion-conducting polymers are known bytradenames such as various formulations of Nafion® (DuPont™),GORE-SELECT® (Gore), Fumapem® (fumatech GmbH), and Aquivion® PFSA(Solvay).

Polymeric Structures

Examples of polymeric structures that can include an ionizable moiety oran ionic moiety and be used as ion-conducting polymers in the MEAsdescribed here are provided below. The ion-conducting polymers may beused as appropriate in any of the MEA layers that include anion-conducting polymer. Charge conduction through the material can becontrolled by the type and amount of charge (e.g., anionic and/orcationic charge on the polymeric structure) provided by theionizable/ionic moieties. In addition, the composition can include apolymer, a homopolymer, a copolymer, a block copolymer, a polymericblend, other polymer-based forms, or other useful combinations ofrepeating monomeric units. As described below, an ion conducting polymerlayer may include one or more of crosslinks, linking moieties, andarylene groups according to various embodiments. In some embodiments,two or more ion conducting polymers (e.g., in two or more ion conductingpolymer layers of the MEA) may be crosslinked.

Non-limiting monomeric units can include one or more of the following:

in which Ar is an optionally substituted arylene or aromatic; Ak is anoptionally substituted alkylene, haloalkylene, aliphatic,heteroalkylene, or heteroaliphatic; and L is a linking moiety (e.g., anydescribed herein) or can be —C(R⁷)(R⁸)—. Yet other non-limitingmonomeric units can include optionally substituted arylene, aryleneoxy,alkylene, or combinations thereof, such as optionally substituted(aryl)(alkyl)ene (e.g., -Ak-Ar— or -Ak-Ar-Ak- or —Ar-Ak-, in which Ar isan optionally substituted arylene and Ak is an optionally substitutedalkylene). One or more monomeric units can be optionally substitutedwith one or more ionizable or ionic moieties (e.g., as describedherein).

One or more monomeric units can be combined to form a polymeric unit.Non-limiting polymeric units include any of the following:

in which Ar, Ak, L, n, and m can be any described herein. In someembodiments, each m is independently 0 or an integer of 1 or more. Inother embodiments, Ar can include two or more arylene or aromaticgroups.

Other alternative configurations are also encompassed by thecompositions herein, such as branched configurations, diblockcopolymers, triblock copolymers, random or statistical copolymers,stereoblock copolymers, gradient copolymers, graft copolymers, andcombinations of any blocks or regions described herein.

Examples of polymeric structures include those according to any one offormulas (I)-(V) and (X)-(XXXIV), or a salt thereof. In someembodiments, the polymeric structures are copolymers and include a firstpolymeric structure selected from any one of formulas (I)-(V) or a saltthereof; and a second polymeric structure including an optionallysubstituted aromatic, an optionally substituted arylene, a structureselected from any one of formulas (I)-(V) and (X)-(XXXIV), or a saltthereof.

In one embodiment, the MW of the ion-conducting polymer is aweight-average molecular weight (Mw) of at least 10,000 g/mol; or fromabout 5,000 to 2,500,000 g/mol. In another embodiment, the MW is anumber average molecular weight (Mn) of at least 20,000 g/mol; or fromabout 2,000 to 2,500,000 g/mol.

In any embodiment herein, each of n, n1, n2, n3, n4, m, m1, m2, or m3is, independently, 1 or more, 20 or more, 50 or more, 100 or more; aswell as from 1 to 1,000,000, such as from 10 to 1,000,000, from 100 to1,000,000, from 200 to 1,000,000, from 500 to 1,000,000, or from 1,000to 1,000,000.

Non-limiting polymeric structures can include the following:

or a salt thereof, wherein:

-   -   each of R⁷, R⁸, R⁹, and R¹⁰ is, independently, an        electron-withdrawing moiety, H, optionally substituted        aliphatic, alkyl, heteroaliphatic, heteroalkylene, aromatic,        aryl, or arylalkylene, wherein at least one of R⁷ or R⁸ can        include the electron-withdrawing moiety or wherein a combination        of R⁷ and R⁸ or R⁹ and R¹⁰ can be taken together to form an        optionally substituted cyclic group;    -   Ar comprises or is an optionally substituted aromatic or arylene        (e.g., any described herein);    -   each of n is, independently, an integer of 1 or more;    -   each of rings a-c can be optionally substituted; and    -   rings a-c, R⁷, R⁸, R⁹, and R¹⁰ can optionally comprise an        ionizable or ionic moiety.

Further non-limiting polymeric structures can include one or more of thefollowing:

or a salt thereof, wherein:

-   -   R⁷ can be any described herein (e.g., for formulas (I)-(V));    -   n is from 1 or more;    -   each L^(8A), L^(B′), and L^(B″) is, independently, a linking        moiety; and    -   each X^(8A), X^(8A′), X^(8A″), X^(B′), and X^(B″) is,        independently, an ionizable or ionic moiety.        Yet other polymeric structures include the following:

or a salt thereof, wherein:

-   -   each of R¹, R², R³, R⁷, R⁸, R⁹, and R¹⁰ is, independently, an        electron-withdrawing moiety, H, optionally substituted        aliphatic, alkyl, heteroaliphatic, heteroalkylene, aromatic,        aryl, or arylalkylene, wherein at least one of R⁷ or R⁸ can        include the electron-withdrawing moiety or wherein a combination        of R⁷ and R⁸ or R⁹ and R¹⁰ can be taken together to form an        optionally substituted cyclic group;    -   each Ak is or comprises an optionally substituted aliphatic,        alkylene, haloalkylene, heteroaliphatic, or heteroalkylene;    -   each Ar is or comprises an optionally substituted arylene or        aromatic;    -   each of L, L′, L2, L3, and L4 is, independently, a linking        moiety;    -   each of n, n1, n2, n3, n4, m, m1, m2, and m3 is, independently,        an integer of 1 or more;    -   q is 0, 1, 2, or more;    -   each of rings a-i can be optionally substituted; and    -   rings a-i, R⁷, R⁸, R⁹, and R¹⁰ can optionally include an        ionizable or ionic moiety.

In particular embodiments (e.g., of formula (XIV) or (XV)), each of thenitrogen atoms on rings a and/or b are substituted with optionallysubstituted aliphatic, alkyl, aromatic, aryl, an ionizable moiety, or anionic moiety. In some embodiments, one or more hydrogen or fluorineatoms (e.g., in formula (XIX) or (XX)) can be substituted to include anionizable moiety or an ionic moiety (e.g., any described herein). Inother embodiments, the oxygen atoms present in the polymeric structure(e.g., in formula XXVIII) can be associated with an alkali dopant (e.g.,K⁺).

In particular examples, Ar, one or more of rings a-i (e.g., rings a, b,f g, h, or i), L, L¹, L², L³, L⁴, Ak, R⁷, R⁸, R⁹, and/or R¹⁰ can beoptionally substituted with one or more ionizable or ionic moietiesand/or one or more electron-withdrawing groups. Yet other non-limitingsubstituents for Ar, rings (e.g., rings a-i), L, Ak, R⁷, R⁸, R⁹, and R¹⁰include one or more described herein, such as cyano, hydroxy, nitro, andhalo, as well as optionally substituted aliphatic, alkyl, alkoxy,alkoxyalkyl, amino, aminoalkyl, aryl, arylalkylene, aryloyl, aryloxy,arylalkoxy, hydroxyalkyl, and haloalkyl.

In some embodiments, each of R¹, R², and R³ is, independently, H,optionally substituted aromatic, aryl, aryloxy, or arylalkylene. Inother embodiments (e.g., of formulas (I)-(V) or (XII)), R⁷ includes theelectron-withdrawing moiety. In yet other embodiments, R⁸, R⁹, and/orR¹⁰ includes an ionizable or ionic moiety.

In one instance, a polymeric subunit can lack ionic moieties.Alternatively, the polymeric subunit can include an ionic moiety on theAr group, the L group, both the Ar and L groups, or be integrated aspart of the L group. Non-limiting examples of ionizable and ionicmoieties include cationic, anionic, and multi-ionic group, as describedherein.

In any embodiment herein, the electron-withdrawing moiety can include orbe an optionally substituted haloalkyl, cyano (CN), phosphate (e.g.,—O(P═O)(OR^(P1))(OR^(P2)) or —O—[P(═O)(OR^(P1))—O]_(P3)—R^(P2)), sulfate(e.g., —O—S(═O)₂(OR^(S1))), sulfonic acid (—SO₃H), sulfonyl (e.g.,—SO₂—CF₃), difluoroboranyl (—BF₂), borono (B(OH)₂), thiocyanato (—SCN),or piperidinium. Yet other non-limiting phosphate groups can includederivatives of phosphoric acid, such as orthophosphoric acid,pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid,trimetaphosphoric acid, and/or phosphoric anhydride, or combinationsthereof.

Yet other polymeric units can include poly(benzimidazole) (PBI),polyphenylene (PP), polyimide (PI), poly(ethyleneimine) (PEI),sulfonated polyimide (SPI), polysulfone (PSF), sulfonated polysulfone(SPSF), poly(ether ether ketone) (PEEK), PEEK with cardo groups(PEEK-WC), polyethersulfone (PES), sulfonated polyethersulfone (SPES),sulfonated poly(ether ether ketone) (SPEEK), SPEEK with cardo groups(SPEEK-WC), poly(p-phenylene oxide) (PPO), sulfonated polyphenyleneoxide (SPPO), ethylene tetrafluoroethylene (ETFE),polytetrafluoroethylene (PTFE), poly(epichlorohydrin) (PECH),poly(styrene) (PS), sulfonated poly(styrene) (SPS), hydrogenatedpoly(butadiene-styrene) (HPBS), styrene divinyl benzene copolymer(SDVB), styrene-ethylene-butylene-styrene (SEBS), sulfonatedbisphenol-A-polysulfone (SP SU), poly(4-phenoxy benzoyl-1,4-phenylene)(PPBP), sulfonated poly(4-phenoxy benzoyl-1,4-phenylene) (SPPBP),poly(vinyl alcohol) (PVA), poly(phosphazene), poly(aryloxyphosphazene),polyetherimide, as well as combinations thereof.

Crosslinking

In some embodiments, crosslinking is present within an ion conductingpolymer layer and/or between ion conducting polymer layers. Crosslinkingwithin a material can be promoted by use of crosslinking reagents. Forinstance, the composition can include polymeric units, and acrosslinking reagent can be used to provide crosslinking betweenpolymeric units. For instance, if the polymeric units (P1 and P2)include a leaving group, then a diamine crosslinking reagent (e.g.,H₂N-Ak-NH₂) can be used to react with the polymeric units by displacingthe leaving group and forming an amino-containing crosslinker within thecomposition (e.g., thereby forming P1-NH-Ak-NH—P2). Crosslinkers can beintroduced by forming a polymer composition and then exposing thecomposition to a crosslinking reagent to form crosslinker.

Depending on the functional group present in the material, thecrosslinking reagent can include a nucleophilic group (e.g., an amine ora hydroxyl) or an electrophilic group (e.g., a carbonyl). Thus,non-limiting crosslinking reagents can include amine-containingreagents, hydroxyl-containing reagents, carboxylic acid-containingreagents, acyl halide-containing reagents, or others. Furthercrosslinking reagents can include:

in which Ak is an optionally substituted aliphatic or alkylene; Ar is anoptionally substituted aromatic or arylene; L is a linking moiety (e.g.,any herein, such as a covalent bond, optionally substituted alkylene,aliphatic, etc.); L3 is an integer that is 2 or more (e.g., 2, 3, 4, 5,6, or more); and X is halo, hydroxyl, optionally substituted amino(e.g., NR^(N1)R^(N2), in which each of R^(N1) and R^(N2) is,independently, H or optionally substituted alkyl), hydroxyl, carboxyl,acyl halide (e.g., —C(O)—R, in which R is halo), carboxyaldehyde (e.g.,—C(O)H), or optionally substituted alkyl. Non-limiting crosslinkingreagents can include terephthalaldehyde, glutaraldehyde, ortho-xylene,para-xylene, meta-xylene, or a multivalent amine, such as diamine,triamine, tetraamine, pentaamine, etc., including 1,6-diaminohexane(hexanediamine), 1,4-diaminobutane, 1,8-diaminooctane,propane-1,2,3-triamine, [1,1′:3′,1″-terphenyl]-4,4″,5′-triamine, andothers.

After reacting the crosslinking reagent, the composition can include oneor more crosslinkers within the composition. If the crosslinking reagentis bivalent, then a crosslinker can be present between two of anycombination of polymeric structures, polymeric units, andionizable/ionic moieties (e.g., between two polymeric units, between twoionizable/ionic moieties, etc.). If the crosslinking reagent istrivalent or of higher n valency, then the crosslinker can be presentbetween any n number of polymeric units, linking moieties, ionizablemoieties, and/or ionic moieties. Non-limiting crosslinkers present inthe composition include those formed after reacting a crosslinkingreagent. Thus, examples of crosslinkers can include:

in which Ak is an optionally substituted aliphatic or an optionallysubstituted alkylene, Ar is an optionally substituted aromatic or anoptionally substituted arylene, L is a linking moiety (e.g., any herein,such as a covalent bond, optionally substituted alkylene, optionallysubstituted aliphatic, etc.), L3 is an integer that is 2 or more (e.g.,2, 3, 4, 5, 6, or more), and X′ is a reacted form of X. In someembodiments, X′ is absent, —O—, —NR^(N1)—, —C(O)—, or -Ak-, in whichR^(N1) is H or optionally substituted alkyl, and Ak is optionallysubstituted alkylene, optionally substituted heteroalkylene, optionallysubstituted aliphatic, or optionally substituted heteroaliphatic.

Ionizable and Ionic Moieties

The polymers described herein include one or more ionizable or ionicmoieties. Such moieties can include an anionic or cationic charge, suchas in an ionic moiety. Alternatively, an ionizable moiety includes afunctional group that can be readily converted into an ionic moiety,such as an ionizable moiety of a carboxy group (—CO₂H) that can bereadily deprotonated to form a carboxylate anion (—CO₂ ⁻). As usedherein, the terms “ionizable” and “ionic” are used interchangeably.

Moieties can be characterized as an acidic moiety (e.g., a moiety can bedeprotonated or can carry a negative charge) or a basic moiety (e.g., amoiety that can be protonated or carry a positive charge). In particularembodiments, the moiety can be a multi-ionic moiety, which can include aplurality of acidic moieties, a plurality of basic moieties, or acombination thereof (e.g., such as in a zwitterionic moiety). Furthermoieties can include a zwitterionic moiety, such as those including ananionic moiety (e.g., hydroxyl or a deprotonated hydroxyl) and acationic moiety (e.g., ammonium).

The ionic moieties herein can be connected to the parent structure byway of one or more linking moieties. Furthermore, a single ionic moietycan be extended from a single linking moiety, or a plurality of ionicmoieties can have one or more linking moieties therebetween. Forinstance, the ionic moiety can have any of the following structures:-L^(A)-X^(A) or -L^(A)-(L^(A′)-X^(A))_(L2) or-L^(A)-X^(A)-L^(A′)-X^(A′)-L^(A″)-X^(A″), in which each L^(A), L^(A′),and L^(A″) is a linking moiety (e.g., any described herein); each X^(A),X^(A′), and X^(A″) includes, independently, an acidic moiety, a basicmoiety, a multi-ionic moiety, a cationic moiety, or an anionic moiety;and L2 is an integer of 1, 2, 3, or more (e.g., from 1 to 20).Non-limiting L^(A) and L^(A1′) can be —(CH₂)_(L1)—, —O(CH₂)_(L1)—,—(CF₂)_(L1)—, —O(CF₂)_(L1)—, or —S(CF₂)_(L1)—, in which L1 is an integerfrom 1 to 3; and X^(A) is any ionizable or ionic moiety describedherein.

Non-limiting ionizable or ionic moieties include carboxy (—CO₂H),carboxylate anion (—CO₂ ⁻), guanidinium cation, sulfo (—SO₂OH),sulfonate anion (—SO₂O⁻), sulfonium cation, sulfate, sulfate anion,phosphono (e.g., —P(═O)(OH)₂), phosphonate anion, phosphate, phosphateanion, phosphonium cation, phosphazenium cation, amino (e.g.,—NR^(N1)R^(N2)), ammonium cation (e.g., aliphatic or aromatic ammonium),heterocyclic cation (e.g., including piperidinium, pyrrolidinium,pyridinium, pyrazolium, imidazolium, quinolinium, isoquinolinium,acridinium, quinolinium, isoquinolinium, acridinium, pyridazinium,pyrimidinium, pyrazinium, phenazinium, 1,4-diazabicyclo[2.2.2]octane(DABCO) cation, 4-methyl-1,4-diazoniabicyclo[2.2.2]octan-1-yl (MAABCO)cation), and 1-benzyl-1,4-diazoniabicyclo[2.2.2] octane (BABCO) cation),or a salt form thereof. Such moieties can be associated with one or morecounterions. For instance, a cationic moiety can be associated with oneor more anionic counterions, and an anionic moiety can be associatedwith one or more cationic counterions.

Arylene Groups

Particular moieties herein (e.g., polymeric units, linking moieties, andothers) can include an optionally substituted arylene. Such arylenegroups include any multivalent (e.g., bivalent, trivalent, tetravalent,etc.) groups having one or more aromatic groups, which can includeheteroaromatic groups. Non-limiting aromatic groups (e.g., for Ar) caninclude any of the following:

in which each of rings a-i can be optionally substituted (e.g., with anyoptional substituents described herein for alkyl or aryl; or with anyionic moiety described herein); L′ is a linking moiety (e.g., anydescribed herein); and each of R′ and R″ is, independently, H,optionally substituted alkyl, optionally substituted aryl, or an ionicmoiety, as described herein. Non-limiting substituents for rings a-iinclude one or more described herein for aryl, such as alkyl, alkoxy,alkoxyalkyl, amino, aminoalkyl, aryl, arylalkylene, aryloyl, aryloxy,arylalkoxy, cyano, hydroxy, hydroxyalkyl, nitro, halo, and haloalkyl. Insome embodiments, L′ is a covalent bond, —O—, —C(O)—, optionallysubstituted alkylene, heteroalkylene, or arylene.

Yet other non-limiting arylene can include phenylene (e.g.,1,4-phenylene, 1,3-phenylene, etc.), biphenylene (e.g.,4,4′-biphenylene, 3,3′-biphenylene, 3,4′-biphenylene, etc.),terphenylene (e.g., 4,4′-terphenylene), diphenyl ether, anthracene(e.g., 9,10-anthracene), naphthalene (e.g., 1,5-naphthalene,1,4-naphthalene, 2,6-naphthalene, 2,7-naphthalene, etc.),tetrafluorophenylene (e.g., 1,4-tetrafluorophenylene,1,3-tetrafluorophenylene), and the like.

Non-limiting examples of linking moieties for arylene include anyherein. In some embodiments, L′ is substituted one or more ionizable orionic moieties described herein. In particular embodiments, L′ isoptionally substituted alkylene. Non-limiting substitutions for L′ caninclude -L^(A)-X^(A), in which L^(A) is a linking moiety (e.g., anydescribed herein, such as, -Ak-, —O-Ak-, -Ak-O—, —Ar—, —O—Ar—, or—Ar—O—, in which Ak is optionally substituted alkylene and Ar isoptionally substituted arylene), and X^(A) is an acidic moiety, a basicmoiety, or a multi-ionic moiety.

Linking Moieties

Particular chemical functionalities herein can include a linking moiety,either between the parent structure and another moiety (e.g., an ionicmoiety) or between two (or more) other moieties. Linking moieties (e.g.,L, L¹, L², L³, L⁴, L^(A), L^(A′), L^(A″), L^(B′), L^(B″), L^(8A), andothers) can be any useful multivalent group, such as multivalent formsof optionally substituted aliphatic, heteroaliphatic, aromatic, orheteroaromatic.

In any embodiment herein, the linking moiety (e.g., L, L¹, L², L³, orL⁴) includes a covalent bond, spirocyclic bond, —O—, —NR^(N1)—, —C(O)—,—C(O)O—, —OC(O)—, —SO₂—, optionally substituted aliphatic, alkylene(e.g., —CH₂—, —C(CH₃)₂—, or —CR₂—, in which R is H, alkyl, orhaloalkyl), alkyleneoxy, haloalkylene (e.g., —CF₂— or —C(CF₃)₂—),hydroxyalkylene, heteroaliphatic, heteroalkylene, aromatic, arylene,aryleneoxy, heterocycle, heterocyclyldiyl, —SO₂—NR^(N1)-Ak-,—(O-Ak)_(L1)-SO₂—NR^(N1)-Ak-, -Ak-, -Ak-(O-Ak)_(L1)-, —(O-Ak)_(L1)-,-(Ak-O)_(L1)—, —C(O)O-Ak-, —Ar—, or —Ar—O—, as well as combinationsthereof. In particular embodiments, Ak is optionally substitutedaliphatic, alkylene, or haloalkylene; R^(N1) is H, optionallysubstituted alkyl, or aryl; Ar is an optionally substituted aromatic orarylene; and L1 is an integer from 1 to 3.

In other embodiments, L is an optionally substituted C₁₋₆ aliphatic,C₁₋₆ alkylene, or C₁₋₆ heteroalkylene. The use of short linkers couldprovide more extensive polymeric networks, as shorter linkers couldminimize self-cyclization reactions.

In some embodiments, the linking moiety is —(CH₂)_(L1)—, —O(CH₂)_(L1)—,—(CF₂)_(L1)—, —O(CF₂)_(L1)—, or —S(CF₂)_(L1)— in which L1 is an integerfrom 1 to 3. In other embodiments, the linking moiety is-Ak-O—Ar-Ak-O-Ak- or -Ak-O—Ar—, in which Ak is optionally substitutedalkylene or haloalkylene, and Ar is an optionally substituted arylene.Non-limiting substituted for Ar includes —SO₂-Ph, in which Ph can beunsubstituted or substituted with one or more halo.

The polymers described above in the with reference to the Table andformulas (I)-(V) and (X)-(XXXIV), including homopolymers and copolymersthereof and which may be optionally crosslinked and may include any ofthe linking moieties, arylene groups, and ionic moieties as describedabove may be used as appropriate in one or more layers of the MEAincluding a cathode catalyst layer, an anode catalyst layer, a polymerelectrolyte membrane (PEM) layer, a cathode buffer layer, and/or ananode buffer layer.

Bipolar MEA for Cox Reduction

In certain embodiments, the MEA includes a bipolar interface with ananion-conducting polymer on the cathode side of the MEA and aninterfacing cation-conducting polymer on the anode side of the MEA. Insome implementations, the cathode contains a first catalyst and ananion-conducting polymer. In certain embodiments, the anode contains asecond catalyst and a cation-conducting polymer. In someimplementations, a cathode buffer layer, located between the cathode andPEM, contains an anion-conducting polymer. In some embodiments, an anodebuffer layer, located between the anode and PEM, contains acation-conducting polymer.

During operation, an MEA with a bipolar interface moves ions through apolymer-electrolyte, moves electrons through metal and/or carbon in thecathode and anode layers, and moves liquids and gas through pores in thelayers.

In embodiments employing an anion-conducting polymer in the cathodeand/or in a cathode buffer layer, the MEA can decrease or block unwantedreactions that produce undesired products and decrease the overallefficiency of the cell. In embodiments employing a cation-conductingpolymer in the anode and/or in an anode buffer layer can decrease orblock unwanted reactions that reduce desired product production andreduce the overall efficiency of the cell.

For example, at levels of electrical potential used for cathodicreduction of CO₂, hydrogen ions may be reduced to hydrogen gas. This isa parasitic reaction; current that could be used to reduce CO₂ is usedinstead to reduce hydrogen ions. Hydrogen ions may be produced byvarious oxidation reactions performed at the anode in a CO₂ reductionreactor and may move across the MEA and reach the cathode where they canbe reduced to produce hydrogen gas. The extent to which this parasiticreaction can proceed is a function of the concentration of hydrogen ionspresent at the cathode. Therefore, an MEA may employ an anion-conductingmaterial in the cathode layer and/or in a cathode buffer layer. Theanion-conducting material at least partially blocks hydrogen ions fromreaching catalytic sites on the cathode. As a result, parasiticproduction of hydrogen gas generation is decreased and the rate of CO orother product production and the overall efficiency of the process areincreased.

Another process that may be avoided is transport of carbonate orbicarbonate ions to the anode, effectively removing CO₂ from thecathode. Aqueous carbonate or bicarbonate ions may be produced from CO₂at the cathode. If such ions reach the anode, they may decompose andrelease gaseous CO₂. The result is net movement of CO₂ from the cathodeto the anode, where it does not get reduced and is lost with oxidationproducts. To prevent the carbonate and bicarbonate ion produced at thecathode from reaching the anode, the polymer-electrolyte membrane and/ora anode buffer layer may include a cation-conducting polymer, which atleast partially blocks the transport of negative ions such asbicarbonate or carbonate ions to the anode.

Thus, in some designs, a bipolar membrane structure raises the pH at thecathode to facilitate CO₂ reduction while a cation-conducting polymersuch as a proton-exchange layer prevents the passage of significantamounts of CO₂, negative ions (e.g. bicarbonate, carbonate), hydrogen,and CO₂ reduction products (e.g., CO, methane, ethylene, alcohols) tothe anode side of the cell.

An example MEA 200 for use in CO_(x) reduction is shown in FIG. 2A. TheMEA 200 has a cathode layer 220 and an anode layer 240 separated by anion-conducting polymer layer 260 that provides a path for ions to travelbetween the cathode layer 220 and the anode layer 240. In certainembodiments, the cathode layer 220 includes an anion-conducting polymerand/or the anode layer 240 includes a cation-conducting polymer. Incertain embodiments, the cathode layer and/or the anode layer of the MEAare porous. The pores may facilitate gas and/or fluid transport and mayincrease the amount of catalyst surface area that is available forreaction.

The ion-conducting layer 260 may include two or three sublayers: apolymer electrolyte membrane (PEM) 265, an optional cathode buffer layer225, and/or an optional anode buffer layer 245. One or more layers inthe ion-conducting layer may be porous. In certain embodiments, at leastone layer is nonporous so that reactants and products of the cathodecannot pass via gas and/or liquid transport to the anode and vice versa.In certain embodiments, the PEM layer 265 is nonporous. Examplecharacteristics of anode buffer layers and cathode buffer layers areprovided elsewhere herein.

FIG. 2B shows an example of an MEA 201 for use in a CRR, according tocertain embodiments. The MEA 201 has a cathode layer 220, an anode layer240 separated by an ion-conducting polymer layer 260. The ion-conductingpolymer layer 260 contains a PEM 265 and a cathode buffer layer 225. Theanode layer 240 and the PEM 265 contain ion-conducting polymers that arecation conductors. In this example, no anode buffer layer is used. ThePEM 265 does not allow for appreciable amounts of bicarbonate to reachthe anode 240.

Another example of an MEA 202 for use in a CRR is shown in FIG. 2C. TheMEA 202 has a cathode layer 220, an anode layer 240, and a PEM 265. Inthis example, no buffer layers are used. In some embodiments, thetransition from a high proton concentration within the PEM 265 to a lowproton concentration in the cathode layer 220 is achieved at theinterface of the cathode layer 220 and the PEM 265 without a bufferlayer between these two layers. In some embodiments, a difference inproton concentration without the buffer layer is achieved by appropriatechoice of the ion-conducting polymers used in the cathode layer 220 andin the PEM 265 and the way in which the ion-conducting polymers mix atthe interface of the layers. For example, in some embodiments, theselayers may use different polymers.

FIG. 3 shows CO₂ electrolyzer 303 configured to receive water and CO₂(e.g., humidified or dry gaseous CO₂) as a reactant at a cathode 305 andexpel CO as a product. Electrolyzer 303 is also configured to receivewater as a reactant at an anode 307 and expel gaseous oxygen.Electrolyzer 303 includes bipolar layers having an anion-conductingpolymer 309 adjacent to cathode 305 and a cation-conducting polymer 311(illustrated as a proton-exchange membrane) adjacent to anode 307.

As illustrated in the magnification inset of a bipolar interface 313 inelectrolyzer 303, the cathode 305 includes an anion exchange polymer(which in this example is the same anion-conducting polymer 309 that isin the bipolar layers) electronically conducting carbon supportparticles 317, and metal nanoparticles 319 supported on the supportparticles. CO₂ and water are transported via pores such as pore 321 andreach metal nanoparticles 319 where they react, in this case withhydroxide ions, to produce bicarbonate ions and reduction reactionproducts (not shown). CO₂ may also reach metal nanoparticles 319 bytransport within anion exchange polymer 315.

Hydrogen ions are transported from anode 307, and through thecation-conducting polymer 311, until they reach bipolar interface 313,where they are hindered from further transport toward the cathode byanion exchange polymer 309. At interface 313, the hydrogen ions mayreact with bicarbonate or carbonate ions to produce carbonic acid(H₂CO₃), which may decompose to produce CO₂ and water. As explainedherein, the resulting CO₂ may be provided in gas phase and should beprovided with a route in the MEA back to the cathode 305 where it can bereduced. The cation-conducting polymer 311 hinders transport of anionssuch as bicarbonate ions to the anode where they could react withprotons and release CO₂, which would be unavailable to participate in areduction reaction at the cathode.

As illustrated, a cathode buffer layer having an anion-conductingpolymer may work in concert with the cathode and its anion-conductivepolymer to block transport of protons to the cathode. While MEAsemploying ion conducting polymers of appropriate conductivity types inthe cathode, the anode, cathode buffer layer, and if present, an anodebuffer layer may hinder transport of cations to the cathode and anionsto the anode, cations and anions may still come in contact in the MEA'sinterior regions, such as in the membrane layer.

As illustrated in FIG. 3 , bicarbonate and/or carbonate ions combinewith hydrogen ions between the cathode layer and the anode layer to formcarbonic acid, which may decompose to form gaseous CO₂. It has beenobserved that MEAs sometime delaminate, possibly due to this productionof gaseous CO₂, which does not have an easy egress path.

The delamination problem can be addressed by employing a cathode bufferlayer having pores. One possible explanation of its effectiveness isthat the pores create paths for the gaseous carbon dioxide to escapeback to the cathode where it can be reduced. In some embodiments, thecathode buffer layer is porous but at least one layer between thecathode layer and the anode layer is nonporous. This can prevent thepassage of gases and/or bulk liquid between the cathode and anode layerswhile still preventing delamination. For example, the nonporous layercan prevent the direct passage of water from the anode to the cathode.The porosity of various layers in an MEA is described further at otherlocations herein.

Examples of Bipolar MEAs

As an example, an MEA includes a cathode layer including a reductioncatalyst and a first anion-conducting polymer (e.g., Sustainion, FumaSepFAA-3, Tokuyama anion exchange polymer), an anode layer including anoxidation catalyst and a first cation-conducting polymer (e.g., PFSApolymer), a membrane layer including a second cation-conducting polymerand arranged between the cathode layer and the anode layer toconductively connect the cathode layer and the anode layer, and acathode buffer layer including a second anion-conducting polymer (e.g.,Sustainion, FumaSep FAA-3, Tokuyama anion exchange polymer) and arrangedbetween the cathode layer and the membrane layer to conductively connectthe cathode layer and the membrane layer. In this example, the cathodebuffer layer can have a porosity between about 1 and 90 percent byvolume, but can additionally or alternatively have any suitable porosity(including, e.g., no porosity). In other examples the cathode bufferlayer can have any suitable porosity (e.g., between 0.01-95%, 0.1-95%,0.01-75%, 1-95%, 1-90%, etc.).

Too much porosity can lower the ionic conductivity of the buffer layer.In some embodiments, the porosity is 20% or below, and in particularembodiments, between 0.1-20%, 1-10%, or 5-10%. Porosity in these rangescan be sufficient to allow movement of water and/or CO₂ without losingionic conductivity. Porosity may be measured as described further below.

In a related example, the membrane electrode assembly can include ananode buffer layer that includes a third cation-conducting polymer, andis arranged between the membrane layer and the anode layer toconductively connect the membrane layer and the anode layer. The anodebuffer layer preferably has a porosity between about 1 and 90 percent byvolume, but can additionally or alternatively have any suitable porosity(including, e.g., no porosity). However, in other arrangements andexamples, the anode buffer layer can have any suitable porosity (e.g.,between 0.01-95%, 0.1-95%, 0.01-75%, 1-95%, 1-90%). As with the cathodebuffer layer, in some embodiments, the porosity is 20% or below, e.g.0.1-20%, 1-10%, or 5-10%

In an example, an anode buffer layer may be used in a MEA having acathode catalyst layer with anion exchange polymer, a cathode bufferlayer with anion-exchange polymer, a membrane with cation-exchangepolymer, and an anode buffer layer with anion-exchange polymer. In sucha structure, the anode buffer layer may be porous to facilitate watertransport to the membrane/anode buffer layer interface. Water will besplit at this interface to make protons that travel through the membraneand hydroxide that travels to the anode catalyst layer. One advantage ofthis structure is the potential use of low-cost water oxidationcatalysts (e.g., NiFeO_(x)) that are only stable in basic conditions.

In another specific example, the membrane electrode assembly includes acathode layer including a reduction catalyst and a firstanion-conducting polymer (e.g., Sustainion, FumaSep FAA-3, Tokuyamaanion exchange polymer), an anode layer including an oxidation catalystand a first cation-conducting polymer, a membrane layer including asecond anion-conducting polymer (e.g., Sustainion, FumaSep FAA-3,Tokuyama anion exchange polymer) and arranged between the cathode layerand the anode layer to conductively connect the cathode layer and theanode layer, and an anode buffer layer including a secondcation-conducting polymer and arranged between the anode layer and themembrane layer to conductively connect the anode layer and the membranelayer.

An MEA containing an anion-exchange polymer membrane and an anode bufferlayer containing cation-exchange polymer may be used for CO reduction.In this case, water would form at the membrane/anode buffer layerinterface. Pores in the anode buffer layer could facilitate waterremoval. One advantage of this structure would be the use of an acidstable (e.g., IrO_(x)) water oxidation catalyst.

In a related example, the membrane electrode assembly can include acathode buffer layer that includes a third anion-conducting polymer andis arranged between the cathode layer and the membrane layer toconductively connect the cathode layer and the membrane layer. The thirdanion-conducting polymer can be the same or different from the firstand/or second anion-conducting polymer. The cathode buffer layerpreferably has a porosity between about 1 and 90 percent by volume butcan additionally or alternatively have any suitable porosity (including,e.g., no porosity). However, in other arrangements and examples, thecathode buffer layer can have any suitable porosity (e.g., between0.01-95%, 0.1-95%, 0.01-75%, 1-95%, 1-90%). In some embodiments, theporosity is 20% or below, and in particular embodiments, between0.1-20%, 1-10%, or 5-10%.

In an example, a cathode catalyst layer composed of Au nanoparticles 4nm in diameter supported on Vulcan XC72R carbon and mixed with TM1(mTPN-1) anion exchange polymer electrolyte (from Orion). Layer is ˜15um thick, Au/(Au+C)=20 wt %, TM1 to catalyst mass ratio of 0.32, massloading of 1.4-1.6 mg/cm2 (total Au+C), estimated porosity of 0.56.Anion-exchange polymer layer composed of TM1 and PTFE particles. PTFE isapproximately 200 nm in diameter. TM1 molecular weight is 30 k-45 k.Thickness of the layer is ˜15 um. PTFE may introduce porosity of about8%. Proton-exchange membrane layer composed of perfluorosulfonic acidpolymer (e.g., Nafion 117). Thickness is approximately 183 um. Membraneforms a continuous layer that prevents significant movement of gas (CO₂,CO, H₂) through the layer. Anode catalyst layer composed of Ir or IrOxnanoparticles (100-200 nm aggregates) that is 10 um thick.

Anion Exchange Membrane-Only MEA for COx Reduction

In some embodiments, an MEA does not contain a cation-conducting polymerlayer. In such embodiments, the electrolyte is not a cation-conductingpolymer and the anode, if it includes an ion-conducting polymer, doesnot contain a cation-conducting polymer. Examples are provided herein.

An anion-exchange membrane (AEM)-only (AEM-only) MEA allows conductionof anions across the MEA. In embodiments in which none of the MEA layershas significant conductivity for cations, hydrogen ions have limitedmobility in the MEA. In some implementations, an AEM-only membraneprovides a high pH environment (e.g., at least about pH 7) and mayfacilitate CO₂ and/or CO reduction by suppressing the hydrogen evolutionparasitic reaction at the cathode. As with other MEA designs, theAEM-only MEA allows ions, notably anions such as hydroxide ions, to movethrough polymer-electrolyte. The pH may be lower in some embodiments; apH of 4 or greater may be high enough to suppress hydrogen evolution.The AEM-only MEA also permits electrons to move to and through metal andcarbon in catalyst layers. In embodiments, having pores in the anodelayer and/or the cathode layer, the AEM-only MEA permits liquids and gasto move through pores.

In certain embodiments, the AEM-only MEA comprises an anion-exchangepolymer electrolyte membrane with an electrocatalyst layer on eitherside: a cathode and an anode. In some embodiments, one or bothelectrocatalyst layers also contain anion-exchange polymer-electrolyte.

In certain embodiments, an AEM-only MEA is formed by depositing cathodeand anode electrocatalyst layers onto porous conductive supports such asgas diffusion layers to form gas diffusion electrodes (GDEs) andsandwiching an anion-exchange membrane between the gas diffusionelectrodes.

In certain embodiments, an AEM-only MEA is used for CO₂ reduction. Theuse of an anion-exchange polymer electrolyte avoids low pH environmentthat disfavors CO₂ reduction. Further, water is transported away fromthe cathode catalyst layer when an AEM is used, thereby preventing waterbuild up (flooding) which can block reactant gas transport in thecathode of the cell.

Water transport in the MEA occurs through a variety of mechanisms,including diffusion and electro-osmotic drag. In some embodiments, atcurrent densities of the CO₂ electrolyzers described herein,electro-osmotic drag is the dominant mechanism. Water is dragged alongwith ions as they move through the polymer electrolyte. For acation-exchange membrane such as Nafion membrane, the amount of watertransport is well characterized and understood to rely on thepre-treatment/hydration of the membrane. Protons move from positive tonegative potential (anode to cathode) with. each carrying 2-4 watermolecules with it, depending on pretreatment. In anion-exchangepolymers, the same type of effect occurs. Hydroxide, bicarbonate, orcarbonate ions moving through the polymer electrolyte will ‘drag’ watermolecules with them. In the anion-exchange MEAs, the ions travel fromnegative to positive voltage, so from cathode to anode, and they carrywater molecules with them, moving water from the cathode to the anode inthe process.

In certain embodiments, an AEM-only MEA is employed in CO reductionreactions. Unlike the CO₂ reduction reaction, CO reduction does notproduce carbonate or bicarbonate anions that could transport to theanode and release valuable reactant.

FIG. 4 illustrates an example construction of a CO_(x) reduction MEA 401having a cathode catalyst layer 403, an anode catalyst layer 405, and ananion-conducting PEM 407. In certain embodiments, cathode catalyst layer403 includes metal catalyst particles (e.g., nanoparticles) that areunsupported or supported on a conductive substrate such as carbonparticles. In some implementations, cathode catalyst layer 403additionally includes an anion-conducting polymer. The metal catalystparticles may catalyze CO_(x) reduction, particularly at pH greater thana threshold pH, which may be pH 4-7, for example, depending on thecatalyst. In certain embodiments, anode catalyst layer 405 includesmetal oxide catalyst particles (e.g., nanoparticles) that areunsupported or supported on a conductive substrate such as carbonparticles. In some implementations, anode catalyst layer 403additionally includes an anion-conducting polymer. Examples of metaloxide catalyst particles for anode catalyst layer 405 include iridiumoxide, nickel oxide, nickel iron oxide, iridium ruthenium oxide,platinum oxide, and the like. Anion-conducting PEM 407 may comprise anyof various anion-conducting polymers such as, for example, HNN5/HNN8 byIonomr, FumaSep by Fumatech, TM1 by Orion, PAP-TP by W7energy,Sustainion by Dioxide Materials, and the like. These and otheranion-conducting polymer that have an ion exchange capacity (IEC)ranging from 1.1 to 2.6 mmol/g, working pH ranges from 0-14, bearablesolubility in some organic solvents, reasonable thermal stability andmechanical stability, good ionic conductivity/ASR and acceptable wateruptake/swelling ratio may be used. The polymers may be chemicallyexchanged to certain anions instead of halogen anions prior to use. Insome embodiments, the anion-conducting polymer may have an IEC of 1 to3.5 mmol; g.

As illustrated in FIG. 4 , CO_(x) such as CO₂ gas may be provided tocathode catalyst layer 403. In certain embodiments, the CO₂ may beprovided via a gas diffusion electrode. At the cathode catalyst layer403, the CO₂ reacts to produce reduction product indicated genericallyas C_(x)O_(y)H_(z). Anions produced at the cathode catalyst layer 403may include hydroxide, carbonate, and/or bicarbonate. These may diffuse,migrate, or otherwise move to the anode catalyst layer 405. At the anodecatalyst layer 405, an oxidation reaction may occur such as oxidation ofwater to produce diatomic oxygen and hydrogen ions. In someapplications, the hydrogen ions may react with hydroxide, carbonate,and/or bicarbonate to produce water, carbonic acid, and/or CO₂. Fewerinterfaces give lower resistance. In some embodiments, a highly basicenvironment is maintained for C2 and C3 hydrocarbon synthesis.

FIG. 5 illustrates an example construction of a CO reduction MEA 501having a cathode catalyst layer 503, an anode catalyst layer 505, and ananion-conducting PEM 507. Overall, the constructions of MEA 501 may besimilar to that of MEA 401 in FIG. 4 . However, the cathode catalyst maybe chosen to promote a CO reduction reaction, which means that differentreduction catalysts would be used in CO and CO₂ reduction embodiments.

In some embodiments, an AEM-only MEA may be advantageous for COreduction. The water uptake number of the AEM material can be selectedto help regulate moisture at the catalyst interface, thereby improvingCO availability to the catalyst. AEM-only membranes can be favorable forCO reduction due to this reason. Bipolar membranes can be more favorablefor CO₂ reduction due to better resistance to CO₂ dissolving andcrossover in basic anolyte media.

In various embodiments, cathode catalyst layer 503 includes metalcatalyst particles (e.g., nanoparticles) that are unsupported orsupported on a conductive substrate such as carbon particles. In someimplementations, cathode catalyst layer 503 additionally includes ananion-conducting polymer. In certain embodiments, anode catalyst layer505 includes metal oxide catalyst particles (e.g., nanoparticles) thatare unsupported or supported on a conductive substrate such as carbonparticles. In some implementations, anode catalyst layer 503additionally includes an anion-conducting polymer. Examples of metaloxide catalyst particles for anode catalyst layer 505 may include thoseidentified for the anode catalyst layer 405 of FIG. 4 . Anion-conductingPEM 507 may comprise any of various anion-conducting polymer such as,for example, those identified for the PEM 407 of FIG. 4 .

As illustrated in FIG. 5 , CO gas may be provided to cathode catalystlayer 503. In certain embodiments, the CO may be provided via a gasdiffusion electrode. At the cathode catalyst layer 503, the CO reacts toproduce reduction product indicated generically as C_(x)O_(y)H_(z).

Anions produced at the cathode catalyst layer 503 may include hydroxideions. These may diffuse, migrate, or otherwise move to the anodecatalyst layer 505. At the anode catalyst layer 505, an oxidationreaction may occur such as oxidation of water to produce diatomic oxygenand hydrogen ions. In some applications, the hydrogen ions may reactwith hydroxide ions to produce water.

While the general configuration of the MEA 501 is similar to that of MEA401, there are certain differences in the MEAs. First, MEAs may bewetter for CO reduction, helping keep the polymer electrolyte hydrated.Also, for CO₂ reduction, a significant amount of CO₂ may be transferredto the anode for an AEM-only MEA such as shown in FIG. 4 . For COreduction, there is less likely to be significant CO gas crossover. Inthis case, the reaction environment could be very basic. MEA materials,including the catalyst, may be selected to have good stability in highpH environment. In some embodiments, a thinner membrane may be used forCO reduction than for CO₂ reduction.

The MEAs disclosed herein may also be used for converting aqueous HCO₃ ⁻solutions to CO gas.

Examples of AEM-Only MEA

1. Copper metal (USRN 40 nm thick Cu, ˜0.05 mg/cm²) was deposited onto aporous carbon sheet (Sigracet 39BC gas diffusion layer) via electronbeam deposition. Ir metal nanoparticles were deposited onto a poroustitanium sheet at a loading of 3 mg/cm² via drop casting. Ananion-exchange membrane from Ionomr (25-50 μm, 80 mS/cm²OH-conductivity, 2-3 mS/cm² HCO₃ ⁻ conductivity, 33-37% water uptake)was sandwiched between the porous carbon and titanium sheets with theelectrocatalyst layers facing the membrane.

2. Sigma Aldrich 80 nm spherical Cu nanoparticles, mixed with FumaSepFAA-3 anion exchange solid polymer electrolyte from Fumatech, FumaSepFAA-3 to catalyst mass ratio of 0.10, setup as described above.

3. The catalyst ink is made up of pure 80 nm Cu nanoparticles (SigmaAldrich) mixed with FumaSep FAA-3 anion exchange solid polymerelectrolyte (Fumatech), FumaSep FAA-3 to catalyst mass ratio of 0.09.The cathode is formed by the ultrasonic spray deposition of the catalystink onto a porous carbon gas diffusion layer (Sigracet 39BB). The anodeis composed of IrOx metal nanoparticles spray-coated onto a poroustitanium sheet. An anion exchange membrane (Ionomr Innovations, Aemion25-50 μm thickness, 80 mS/cm² OH-conductivity, 2-3 mS/cm²HCO₃-conductivity, 33-37% water uptake) is sandwiched between the Cucatalyst-coated carbon gas diffusion layer cathode and IrOx-coatedporous titanium anode, with the Cu catalyst-coated side facing themembrane to compose the MEA.

US Patent Application Publication No. US 2017/0321334, published Nov. 9,2017 [OPUSP001B] and US Patent Application Publication No. 20190226103,published Jul. 25, 2019 [OPUSP005] and U.S. Provisional Application62/331,387, filed May 3, 2016, which describe various features andexamples of MEAs and CRRs, are incorporated herein by reference in theirentireties. All publications referred to herein are incorporated byreference in their entireties as if fully set forth herein.

Individual Layers of MEA Cathode Catalyst Layer—General Structure

As indicated above, the cathode of the MEA, which is also referred to asthe cathode layer or cathode catalyst layer, facilitates COx conversion.It is a porous layer containing catalysts for COx reduction reactions.

In some embodiments, the cathode catalyst layer contains a blend ofreduction catalyst particles, electronically-conductive supportparticles that provide support for the reduction catalyst particles, anda cathode ion-conducting polymer. In some embodiments, the reductioncatalyst particles are blended with the cathode ion-conducting polymerwithout a support.

Examples of materials that can be used for the reduction catalystparticles include, but are not limited, to transition metals such as V,Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Au, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W,Re, Ir, Pt, and Hg, and combinations thereof, and/or any other suitablematerials. Other catalyst materials can include alkali metals, alkalineearth metals, lanthanides, actinides, and post transition metals, suchas Sn, Si, Ga, Pb, Al, Tl, Sb, Te, Bi, Sm, Tb, Ce, Nd and In orcombinations thereof, and/or any other suitable catalyst materials. Thechoice of catalyst depends on the particular reaction performed at thecathode of the CRR.

Catalysts can be in the form of nanoparticles that range in size fromapproximately 1 to 100 nm or particles that range in size fromapproximately 0.2 to 10 nm or particles in the size range ofapproximately 1-1000 nm or any other suitable range. In addition tonanoparticles and larger particles, films and nanostructured surfacesmay be used.

If used, the electronically-conductive support particles in the cathodecan be carbon particles in various forms. Other possible conductivesupport particles include boron-doped diamond or fluorine-doped tinoxide. In one arrangement, the conductive support particles are Vulcancarbon. The conductive support particles can be nanoparticles. The sizerange of the conductive support particles is between approximately 20 nmand 1000 nm or any other suitable range. It is especially useful if theconductive support particles are compatible with the chemicals that arepresent in the cathode when the CRR is operating, are reductivelystable, and have a high hydrogen production overpotential so that theydo not participate in any electrochemical reactions.

For composite catalysts such as Au/C, example metal nanoparticle sizesmay range from about 1-100 nm, e.g., 2 nm-20 nm and the carbon size maybe from about 20-200 nm as supporting materials. For pure metal catalystsuch as Ag or Cu, the particles have a broad range from 2 nm to 500 nmin term of crystal grain size. The agglomeration could be even larger tomicrometer range.

In general, such conductive support particles are larger than thereduction catalyst particles, and each conductive support particle cansupport many reduction catalyst particles. FIG. 6 is a schematic drawingthat shows a possible morphology for two different kinds of catalystssupported on a catalyst support particle 610, such as a carbon particle.Catalyst particles 630 of a first type and second catalyst particles 650of a second type are attached to the catalyst support particle 610. Invarious arrangements, there is only one type of catalyst particle orthere are more than two types of catalyst particles attached to thecatalyst support particle 610.

Using two types of catalysts may be useful in certain embodiments. Forexample, one catalyst may be good at one reaction (e.g., CO₂→CO) and thesecond good at another reaction (e.g., CO→CH₄). Overall, the catalystlayer would perform the transformation of CO₂ to CH₄, but differentsteps in the reaction would take place on different catalysts.

The electronically-conductive support may also be in forms other thanparticles, including tubes (e.g., carbon nanotubes) and sheets (e.g.,graphene). Structures having high surface area to volume are useful toprovide sites for catalyst particles to attach.

In addition to reduction catalyst particles andelectronically-conductive support particles, the cathode catalyst layermay include an ion conducting polymer. There are tradeoffs in choosingthe amount of cathode ion-conducting polymer in the cathode. It can beimportant to include enough cathode ion-conducting polymer to providesufficient ionic conductivity. But it is also important for the cathodeto be porous so that reactants and products can move through it easilyand to maximize the amount of catalyst surface area that is availablefor reaction. In various arrangements, the cathode ion-conductingpolymer makes up somewhere in the range between 30 and 70 wt %, between20 and 80 wt %, or between 10 and 90 wt %, of the material in thecathode layer, or any other suitable range. The wt % of ion-conductingpolymer in the cathode is selected to result in the cathode layerporosity and ion-conductivity that gives the highest current density forCO_(x) reduction. In some embodiments, it may be between 20 and 60 wt. %or between 20 and 50 wt. %. Example thicknesses of the cathode catalystlayer range from about 80 nm-300 μm.

In addition to the reduction catalyst particles, cathode ion conductingpolymer, and if present, the electronically-conductive support, thecathode catalyst layer may include other additives such as PTFE.

In addition to polymer:catalyst mass ratios, the catalyst layer may becharacterized by mass loading (mg/cm²), and porosity. Porosity may bedetermined by a various manners. In one method, the loading of eachcomponent (e.g., catalyst, support, and polymer) is multiplied by itsrespective density. These are added together to determine the thicknessthe components take up in the material. This is then divided by thetotal known thickness to obtain the percentage of the layer that isfilled in by the material. The resulting percentage is then subtractedfrom 1 to obtain the percentage of the layer assumed to be void space(e.g., filled with air or other gas or a vacuum), which is the porosity.Methods such as mercury porosimetry or image processing on TEM imagesmay be used as well.

The catalyst layer may also be characterized by its roughness. Thesurface characteristics of the catalyst layer can impact the resistancesacross the membrane electrode assembly. Excessively rough catalystlayers can potentially lead to interfacial gaps between the catalyst andthe microporous layer. These gaps hinder the continuous pathway forelectron transfer from the current collector to the catalytic area,thus, increasing contact resistances. Interfacial gaps may also serve aslocations for water accumulation that is detrimental to mass transportof reactants and products. On the other hand, extremely smooth surfacesmay suffer from poor adhesion between layers. Catalyst layer roughnessmay influence electrical contact resistances and concentrationpolarization losses. Surface roughness can be measured using differenttechniques (e.g. mechanical stylus method, optical profilometry, oratomic force microscopy) and is defined as the high-frequency, shortwavelength component of a real surface. Arithmetic mean height, S_(a),is a parameter that is commonly used to evaluate the surface roughness.Numerically, it is calculated by integrating the absolute height ofvalleys and peaks on the surface relative to the mean plane over theentire geometric area of the sample. Catalyst layer S_(a) values between0.50-1.10 μm or 0.70-0.90 μm may be used in some embodiments.

Examples of cathode catalyst layers for CO, methane, andethylene/ethanol productions are given below.

-   -   CO production: Au nanoparticles 4 nm in diameter supported on        Vulcan XC72R carbon and mixed with TM1 anion exchange polymer        electrolyte from Orion. Layer is about 15 μm thick,        Au/(Au+C)=30%, TM1 to catalyst mass ratio of 0.32, mass loading        of 1.4-1.6 mg/cm², estimated porosity of 0.47    -   Methane production: Cu nanoparticles of 20-30 nm size supported        on Vulcan XC72R carbon, mixed with FAA-3 anion exchange solid        polymer electrolyte from Fumatech. FAA-3 to catalyst mass ratio        of 0.18. Estimated Cu nanoparticle loading of ˜7.1 μg/cm²,        within a wider range of 1-100 μg/cm²    -   Ethylene/ethanol production: Cu nanoparticles of 25-80 nm size,        mixed with FAA-3 anion exchange solid polymer electrolyte from        Fumatech. FAA-3 to catalyst mass ratio of 0.10. Deposited either        on Sigracet 39BC GDE for pure AEM or onto the        polymer-electrolyte membrane. Estimated Cu nanoparticle loading        of 270 μg/cm².    -   Bipolar MEA for methane production: The catalyst ink is made up        of 20 nm Cu nanoparticles supported by Vulcan carbon (Premetek        40% Cu/Vulcan XC-72) mixed with FAA-3 anion exchange solid        polymer electrolyte (Fumatech), FAA-3 to catalyst mass ratio of        0.18. The cathode is formed by the ultrasonic spray deposition        of the catalyst ink onto a bipolar membrane including FAA-3        anion exchange solid polymer electrolyte spray-coated on Nafion        (PFSA) 212 (Fuel Cell Etc) membrane. The anode is composed of        IrRuOx which is spray-coated onto the opposite side of the        bipolar membrane, at a loading of 3 mg/cm². A porous carbon gas        diffusion layer (Sigracet 39BB) is sandwiched to the Cu        catalyst-coated bipolar membrane to compose the MEA.    -   Bipolar MEA for ethylene production: The catalyst ink is made up        of pure 80 nm Cu nanoparticles (Sigma Aldrich) mixed with FAA-3        anion exchange solid polymer electrolyte (Fumatech), FAA-3 to        catalyst mass ratio of 0.09. The cathode is formed by the        ultrasonic spray deposition of the catalyst ink onto a bipolar        membrane including FAA-3 anion exchange solid polymer        electrolyte spray-coated on Nafion (PFSA) 115 (Fuel Cell Etc)        membrane. The anode is composed of IrRuOx which is spray-coated        onto the opposite side of the bipolar membrane, at a loading of        3 mg/cm². A porous carbon gas diffusion layer (Sigracet 39BB) is        sandwiched to the Cu catalyst-coated bipolar membrane to compose        the MEA.    -   CO production: Au nanoparticles 4 nm in diameter supported on        Vulcan XC72R carbon and mixed with TM1 anion exchange polymer        electrolyte from Orion. Layer is about 14 micron thick,        Au/(Au+C)=20%. TM1 to catalyst mass ratio of 0.32, mass loading        of 1.4-1.6 mg/cm², estimated porosity of 0.54 in the catalyst        layer.    -   CO production: Au nanoparticles 45 nm in diameter supported on        Vulcan XC72R carbon and mixed with TM1 anion exchange polymer        electrolyte from Orion. Layer is about 11 micron thick,        Au/(Au+C)=60%. TM1 to catalyst mass ratio of 0.16, mass loading        of 1.1-1.5 mg/cm², estimated porosity of 0.41 in the catalyst        layer.    -   CO production: Au nanoparticles 4 nm in diameter supported on        Vulcan XC72R carbon and mixed with TM1 anion exchange polymer        electrolyte from Orion. Layer is about 25 micron thick,        Au/(Au+C)=20%. TM1 to catalyst mass ratio of 0.32, mass loading        of 1.4-1.6 mg/cm², estimated porosity of 0.54 in the catalyst        layer.

The functions, materials, and structures of the components of thecathode catalyst layer are described further below.

Cathode Catalyst Layer—Functions

A primary function of the cathode catalyst layer is to provide acatalyst for CO_(x) reduction. An example reaction is:

CO₂+2H⁺+2e−→CO+H₂O.

The cathode catalyst layer also has a number of other functions thatfacilitate CO_(x) conversion. These include water management, gastransport, reactant delivery to the metal catalyst, product removal,stabilizing the particulate structure of the metal catalyst, electronicand ionic conduction to the metal catalyst, and mechanical stabilitywithin the MEA.

Certain functions and challenges are particular to CRRs and are notfound in MEA assemblies for other applications such as fuel cells orwater electrolyzers. These challenges include that the cathode catalystlayer of the MEA transports gas (e.g., CO₂ or CO) in and gas (e.g.,ethylene, methane, CO) or liquid (e.g., ethanol) out. The cathodecatalyst layer also prevents accumulation of water that can block gastransport. Further, catalysts for COx reduction are not as developed ascatalysts like platinum that can be used in hydrogen fuel cells. As aresult, the COx reduction catalysts are generally less stable. Thesefunctions, their particular challenges, and how they can be addressedare described below.

Water Management (Cathode Catalyst Layer)

The cathode catalyst layer facilitates movement of water to prevent itfrom being trapped in the cathode catalyst layer. Trapped water canhinder access of CO_(x) to the catalyst and/or hinder movement ofreaction product out of the cathode catalyst layer.

Water management challenges are in many respects unique to CRRs. Forexample, compared to a PEM fuel cell's oxygen electrode, a CRR uses amuch lower gas flow rate. A CRR also may use a lower flow rate toachieve a high utilization of the input CO_(x). Vapor phase waterremoval is determined by the volumetric gas flow, thus much less vaporphase water removal is carried out in a CRR. A CRR may also operate athigher pressure (e.g., 100 psi-450 psi) than a fuel cell; at higherpressure the same molar flow results in lower volumetric flow and lowervapor phase water removal. As a result, liquid water in MEA of a CRR ispresent to be removed. For some MEAs, the ability to remove vapor phasewater is further limited by temperature limits not present in fuelcells. For example, CO₂ to CO reduction may be performed at about 50° C.and ethylene and methane production may be performed at 20° C.-25° C.This is compared to typical operating temperatures of 80° C. to 120° C.for fuel cells. As a result, there is more liquid phase water to remove.

Properties that affect ability of the cathode catalyst layer to removewater include porosity; pore size; distribution of pore sizes;hydrophobicity; the relative amounts of ion conducting polymer, metalcatalyst particles, and electronically-conductive support; the thicknessof the layer; the distribution of the catalyst throughout the layer; andthe distribution of the ion conducting polymer through the layer andaround the catalyst.

A porous layer allows an egress path for water. In some embodiments, thecathode catalyst layer has a pore size distribution that includes poreshaving sizes of 1 nm-100 nm and pores having sizes of at least 1 micron.This size distribution can aid in water removal. The porous structurescould be formed by one or more of: pores within the carbon supportingmaterials; stacking pores between stacked spherical carbonnanoparticles; secondary stacking pores between agglomerated carbonspheres (micrometer scale); or inert filler (e.g., PTFE) introducedporous with the interface between the PTFE and carbon also creatingirregular pores ranging from hundreds of nm to micrometers.

The cathode catalyst layer may have a thickness that contributes towater management. Using a thicker layer allows the catalyst and thus thereaction to be distributed in a larger volume. This spreads out thewater distribution and makes it easier to manage.

Ion-conducting polymers having non-polar, hydrophobic backbones may beused in the cathode catalyst layer. In some embodiments, the cathodecatalyst layer may include a hydrophobic polymer such as PTFE inaddition to the ion-conducting polymer. In some embodiments, theion-conducting polymer may be a component of a co-polymer that alsoincludes a hydrophobic polymer. In some embodiments, the ion-conductingpolymer has hydrophobic and hydrophilic regions. The hydrophilic regionscan support water movement and the hydrophobic regions can support gasmovement.

Gas Transport (Cathode Catalyst Layer)

The cathode catalyst layer is structured for gas transport.Specifically, CO_(x) is transported to the catalyst and gas phasereaction products (e.g., CO, ethylene, methane, etc.) is transported outof the catalyst layer.

Certain challenges associated with gas transport are unique to CRRs. Gasis transported both in and out of the cathode catalyst layer—CO_(x) inand products such as CO, ethylene, and methane out. In a PEM fuel cell,gas (O₂ or H₂) is transported in but nothing or product water comes out.And in a PEM water electrolyzer, water is the reactant with O₂ and H₂gas products.

Operating conditions including pressures, temperature, and flow ratethrough the reactor affect the gas transport. Properties of the cathodecatalyst layer that affect gas transport include porosity; pore size anddistribution; layer thickness; and ionomer distribution.

In some embodiments, the ionomer-catalyst contact is minimized. Forexample, in embodiments that use a carbon support, the ionomer may forma continuous network along the surface of the carbon with minimalcontact with the catalyst. The ionomer, support, and catalyst may bedesigned such that the ionomer has a higher affinity for the supportsurface than the catalyst surface. This can facilitate gas transport toand from the catalyst without being blocked by the ionomer, whileallowing the ionomer to conduct ions to and from the catalyst.

Ionomer (Cathode Catalyst Layer)

The ionomer may have several functions including holding particles ofthe catalyst layer together and allowing movement of ions through thecathode catalyst layer. In some cases, the interaction of the ionomerand the catalyst surface may create an environment favorable for CO_(x)reduction, increasing selectivity to a desired product and/or decreasingthe voltage required for the reaction. Importantly, the ionomer is anion-conducting polymer to allow for the movement of ions through thecathode catalyst layer. Hydroxide, bicarbonate, and carbonate ions, forexample, are moved away from the catalyst surface where the CO_(x)reduction occurs. In the description below, the ionomer in the cathodecatalyst layer can be referred to as a first ion-conducting polymer.

The first ion-conducting polymer can comprise at least oneion-conducting polymer that is an anion-conductor. This can beadvantageous because it raises the pH compared to a proton conductor.

In some embodiments, the first ion-conducting polymer can comprise oneor more covalently-bound, positively-charged functional groupsconfigured to transport mobile negatively-charged ions. The firstion-conducting polymer can be selected from the group consisting ofaminated tetramethyl polyphenylene;poly(ethylene-co-tetrafluoroethylene)-based quaternary ammonium polymer;quaternized polysulfone), blends thereof, and/or any other suitableion-conducting polymers. The first ion-conducting polymer can beconfigured to solubilize salts of bicarbonate or hydroxide.

In some embodiments, the first ion-conducting polymer can comprise atleast one ion-conducting polymer that is a cation-and-anion-conductor.The first ion-conducting polymer can be selected from the groupconsisting of polyethers that can transport cations and anions andpolyesters that can transport cations and anions. The firstion-conducting polymer can be selected from the group consisting ofpolyethylene oxide, polyethylene glycol, polyvinylidene fluoride, andpolyurethane.

A cation-and-anion conductor will raise pH (compared to a pure cationconductor.) Further, in some embodiments, it may be advantageous to usea cation-and-anion conductor to promote acid base recombination in alarger volume instead of at a 2D interface of anion-conducting polymerand cation conducting polymer. This can spread out water and CO₂formation, heat generation, and potentially lower the resistance of themembrane by decreasing the barrier to the acid-base reaction. All ofthese may be advantageous in helping avoid the buildup of products,heat, and lowering resistive losses in the MEA leading to a lower cellvoltage.

A typical anion-conducting polymer has a polymer backbone withcovalently bound positively charged functional groups appended. Thesemay include positively charged nitrogen groups in some embodiments. Insome embodiments, the polymer backbone is non-polar, as described above.The polymer may be any appropriate molecular weight, e.g., 25,000g/mol-150,000 g/mol, though it will be understood that polymers outsidethis range may be used.

Particular challenges for ion-conducting polymers in CRR's include thatCO₂ can dissolve or solubilize polymer electrolytes, making them lessmechanically stable, prone to swelling, and allowing the polymer to movemore freely. This makes the entire catalyst layer andpolymer-electrolyte membrane less mechanically stable. In someembodiments, polymers that are not as susceptible to CO₂ plasticizationare used. Also, unlike for water electrolyzers and fuel cells,conducting carbonate and bicarbonate ions is a key parameter for CO₂reduction.

The introduction of polar functional groups, such as hydroxyl andcarboxyl groups which can form hydrogen bonds, leads topseudo-crosslinked network formation. Cross-linkers like ethylene glycoland aluminum acetylacetonate can be added to reinforce the anionexchange polymer layer and suppress polymer CO₂ plasticization.Additives like polydimethylsiloxane copolymer can also help mitigate CO₂plasticization.

According to various embodiments, the ion-conducting polymer may have abicarbonate ionic conductivity of at least 6 mS/cm, or in someembodiments at least 12 mS/cm, is chemically and mechanically stable attemperatures 80° C. and lower, and soluble in organic solvents usedduring fabrication such as methanol, ethanol, and isoproponal. Theion-conducting polymer is stable (chemically and has stable solubility)in the presence of the CO_(x) reduction products. The ion-conductingpolymer may also be characterized by its ion exchange capacity, thetotal of active sites or functional groups responsible for ion exchange,which may range from 2.1 mmol/g-2.6 mmol/g in some embodiments. In someembodiments, ion-conducting polymers having lower IECs such as greaterthan 1 or 1.5 mmol/g may be used.

Examples of anion-conducting polymers are given above in above table asClass A ion-conducting polymers. A particular example of ananion-conducting polymer is Orion mTPN1 (also referred to herein asOrion TM1), which has m-triphenyl fluori-alkylene as backbone andtrimethylamonium (TMA+) as cation group. The chemical structure is shownbelow.

Additional examples include anion exchange membranes produced byFumatech and Ionomr. Fumatech FumaSep FAA-3 ionomers come in Br— form.Anion exchange polymer/membrane based on polybenzimidazole produced byIonomr comes in I— form as AF-1-HNN8-50-X.

The as-received polymer may be prepared by exchanging the anion (e.g.,I⁻, Br⁻, etc.) with bicarbonate.

Also, as indicated above, in certain embodiments the ionomer may be acation-and-anion-conducting polymer. Examples are given in the abovetable as Class B ion-conducting polymers.

Metal Catalyst (Cathode Catalyst Layer)

The metal catalyst catalyzes the COx reduction reaction(s). The metalcatalyst is typically nanoparticles, but larger particles, films, andnanostructured surfaces may be used in some embodiments. The specificmorphology of the nanoparticles may expose and stabilize active sitesthat have greater activity.

The metal catalyst is often composed of pure metals (e.g., Cu, Au, Ag),but specific alloys or other bimetallic systems may have high activityand be used for certain reactions. The choice of catalyst may be guidedby the desired reaction. For example, for CO production, Au may be used;for methane and ethylene production, Cu may be used. Other metalsincluding Ag, alloys, and bimetallic systems may be used. CO₂ reductionhas a high overpotential compared to other well-known electrochemicalreactions such as hydrogen evolution and oxygen evolution on knowncatalysts. Small amounts of contaminants can poison catalysts for CO₂conversion. And as indicated above, metal catalysts such as Cu, Au, andAg are less developed than catalysts such as platinum used in hydrogenfuel cells.

Different metal catalyst materials may be chosen at least in part basedon the desired product and MEA operation. For example, the 1D nanowire(rightmost image) has a higher selectivity for ethylene production whiletriangular Cu nanoplates (second from left) show higher selectivity formethane. The nanocubes (far left) show good selectivity for ethylene inan AEM MEA. Gold nanoparticles with a narrow size distribution (e.g.,2-6 nm) and uniform distribution on carbon surface resulted in highercurrent efficiency and durability.

Metal catalyst properties that affect the cathode catalyst layerperformance include size, size distribution, uniformity of coverage onthe support particles, shape, loading (characterized as weight ofmetal/weight of metal+weight of carbon or as mass of particles pergeometric area of catalyst layer), surface area (actual metal catalystsurface area per volume of catalyst layer), purity, and the presence ofpoisoning surface ligands from synthesis.

Nanoparticles may be synthesized by any appropriate method, such as forexample, described in Phan et al., “Role of Capping Agent in WetSynthesis of Nanoparticles,” J. Phys. Chem. A 2018, 121, 17, 3213-3219;Bakshi “How Surfactants Control Crystal Growth of Nanomaterials,” Cryst.Growth Des. 2016, 16, 2, 1104-1133; and Morsy “Role of Surfactants inNanotechnology and Their Applications,” Int. J. Curr. Microbiol. App.Sci. 2014, 3, 5, 237-260, which are incorporated by reference herein.

In some embodiments, metal nanoparticles are provided without thepresence of poisoning surface ligands. This may be achieved by using theionomer as a ligand to direct the synthesis of nanocrystal catalysts asillustrated in FIG. 8 . The surface of the metal nanocatalysts aredirectly connected with ionically conductive ionomer. This avoids havingto treat the catalyst surface to allow ionomer contact with the metaland improves the contact.

The metal catalyst may be disposed on a carbon support in someembodiments. For CO production, examples include Premetek 20 wt % Ausupported on Vulcan XC-72R carbon with 4-6 nm Au particle size and 30%Au/C supported on Vulcan XC-72R with 5-7 nm Au particle size. Formethane, examples include Premetek 20 wt % Cu supported on Vulcan XC-72Rcarbon with 20-30 nm Cu particle size. In some embodiments, the metalcatalyst may be unsupported. For ethylene production, examples ofunsupported metal catalysts include SigmaAldrich unsupported Cu 80 nmparticle size and ebeam or sputter deposited thin Cu layer of 10 nm to100 nm.

Support (Cathode Catalyst Layer)

The support of the cathode catalyst layer has several functions. Itstabilizes metal nanoparticles to prevent them from agglomerating anddistributes the catalytic sites throughout the catalyst layer volume tospread out loss of reactants and formation of products. It also forms anelectrically conductive pathway to metal nanoparticles. Carbonparticles, for example, pack together such that contacting carbonparticles provide the electrically conductive pathway. Void spacebetween the particles forms a porous network that gas and liquids cantravel through.

In some embodiments, carbon supports developed for fuel cells can beused. Many different types have been developed; these are typically 50nm-500 nm in size, and can be obtained in different shapes (spheres,nanotubes, sheets (e.g., graphene)), porosities, surface area pervolume, electrical conductivity, functional groups (N-doped, O-doped,etc).

The support may be hydrophobic and have affinity to the metalnanoparticle.

Examples of carbon blacks that can be used include:

-   -   Vulcan XC-72R—Density of 256 mg/cm2, 30-50 nm    -   Ketjen Black—Hollow structure, Density of 100-120 mg/cm2, 30-50        nm    -   Printex Carbon, 20-30 nm

Anode Catalyst Layer

The anode of the MEA, which is also referred to as the anode layer oranode catalyst layer, facilitates oxidation reactions. It is a porouslayer containing catalysts for oxidation reactions. Examples ofreactions are:

2H₂O→4H++4e ⁻+O₂ (in acidic environment of proton exchange polymerelectrolyte—bipolar membrane); or

4OH⁻→4e ⁻+O₂+2H₂O (in basic environment of anion exchange polymerelectrolyte)

The oxidation of other materials, such as hydrocarbons to make CO₂ orchloride ions to make chlorine gas, or hydrogen gas to make hydrogenions, may also be performed.

In some embodiments, with reference to FIG. 2 , the anode 240 contains ablend of oxidation catalyst and an anode ion-conducting polymer. Thereare a variety of oxidation reactions that can occur at the anodedepending on the reactant that is fed to the anode and the anodecatalyst(s). The table below includes some examples of oxidationreactions that can occur at the anode and example catalysts that cansupport those reactions.

Feed Anode Material Oxidation Reaction Exemplary Catalysts Hydrogen H₂→2H⁺ + 2e⁻ Pt, Ni, Ru, other transition metals, and alloys and oxidesthereof Methane CH₄ + H₂O → CH₃OH + Pd, Pd alloys and oxides 2H⁺ + 2e⁻thereof Methane CH₄ + 2H₂O → CO₂ + Pt, Au, Pd, and alloys and 8H⁺ + 8e⁻oxides thereof Ammonia 2NH₃ → N₂ + 6H⁺ + 6e⁻ Ru, Pt and oxides thereofWater 2H₂O → O₂ + 4H⁺ + 4e⁻ Ir, IrRu, PtIr, Pt, Au, Ni, NiFe, Mn,Stainless steel and oxides thereof

In one arrangement, the oxidation catalyst is selected from the groupconsisting of metals and oxides of Ir, Pt, Ni, Ru, Pd, Au, and alloysthereof, IrRu, PtIr, Ni, NiFe, stainless steel, and combinationsthereof. The oxidation catalyst can further contain conductive supportparticles selected from the group consisting of carbon, boron-dopeddiamond, and titanium.

The oxidation catalyst can be in the form of a structured mesh or can bein the form of particles. If the oxidation catalyst is in the form ofparticles, the particles can be supported by electronically-conductivesupport particles. The conductive support particles can benanoparticles. It is especially useful if the conductive supportparticles are compatible with the chemicals that are present in theanode 240 when the CRR is operating and are oxidatively stable so thatthey do not participate in any electrochemical reactions. It can beuseful if the conductive support particles are chosen with the voltageand the reactants at the anode in mind. In some arrangements, theconductive support particles are titanium, which is well-suited for highvoltages. In other arrangements, the conductive support particles arecarbon, which can be most useful at low voltages. In general, suchconductive support particles are larger than the oxidation catalystparticles, and each conductive support particle can support manyoxidation catalyst particles. An example of such an arrangement is shownin FIG. 6 and is discussed above with respect to the cathode catalystlayer. In one arrangement, the oxidation catalyst is iridium rutheniumoxide. Examples of other materials that can be used for the oxidationcatalyst include, but are not limited to, those listed above. It shouldbe understood that many of these metal catalysts can be in the form ofoxides, especially under reaction conditions.

In some embodiments, the MEA has an anode layer comprising oxidationcatalyst and a second ion-conducting polymer. The second ion-conductingpolymer can comprise one or more polymers that contain covalently-bound,negatively-charged functional groups configured to transport mobilepositively-charged ions. The second ion-conducting polymer can beselected from the group consisting of ethanesulfonyl fluoride,2-[1-[difluoro-[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2,-tetrafluoro-,with tetrafluoroethylene,tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acidcopolymer, other perfluorosulfonic acid polymers and blends thereof.Examples of cation-conducting polymers include e.g., Nafion 115, Nafion117, and/or Nafion 211.

There are tradeoffs in choosing the amount of ion-conducting polymer inthe anode. It is important to include enough anode ion-conductingpolymer to provide sufficient ionic conductivity. But it is alsoimportant for the anode to be porous so that reactants and products canmove through it easily, and to maximize the amount of catalyst surfacearea that is available for reaction. In various arrangements, theion-conducting polymer in the anode makes up approximately 50 wt % ofthe layer or between approximately 5 and 20 wt %, 10 and 90 wt %,between 20 and 80 wt %, between 25 and 70 wt %, or any suitable range.It is especially useful if the anode 240 can tolerate high voltages,such as voltages above about 1.2 V vs. a reversible hydrogen electrode.It is especially useful if the anode 240 is porous in order to maximizethe amount of catalyst surface area available for reaction and tofacilitate gas and liquid transport.

In one example of a metal catalyst, Ir or IrOx particles (100-200 nm)and Nafion ionomer form a porous layer approximately 10 μm thick. Metalcatalyst loading is approximately 0.5-3 g/cm².

In some embodiments, NiFeOx or NiO_(x) is used for basic reactions.

PEM (MEA Layer Description)

The MEAS include a polymer electrolyte membrane (PEM) disposed betweenand conductively coupled to the anode catalyst layer and the cathodecatalyst layer. Referring to FIG. 2 , the polymer electrolyte membrane265 has high ionic conductivity (greater than about 1 mS/cm), and ismechanically stable. Mechanical stability can be evidenced in a varietyof ways such as through high tensile strength, modulus of elasticity,elongation to break, and tear resistance. Many commercially-availablemembranes can be used for the polymer electrolyte membrane 265. Examplesinclude, but are not limited to, various Nafion® formulations,GORE-SELECT, FumaPEM® (PFSA) (FuMA-Tech GmbH), and Aquivion (PFSA)(Solvay).

In one arrangement, the PEM comprises at least one ion-conductingpolymer that is a cation-conductor. The third ion-conducting polymer cancomprise one or more covalently-bound, negatively-charged functionalgroups configured to transport mobile positively-charged ions. The thirdion-conducting polymer can be selected from the group consisting ofethanesulfonyl fluoride,2-[1-[difluoro-[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2,-tetrafluoro-,with tetrafluoroethylene,tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acidcopolymer, other perfluorosulfonic acid polymers and blends thereof.

Cathode Buffer Layer (MEA Layer Description)

Referring to FIG. 2 , it is important to note that when the polymerelectrolyte membrane 265 is a cation conductor and is conductingprotons, it contains a high concentration of protons during operation ofthe CRR, while the cathode 220 operates best when a low concentration ofprotons is present. It can be useful to include a cathode buffer layer225 between the polymer electrolyte membrane 265 and the cathode 220 toprovide a region of transition from a high concentration of protons to alow concentration of protons. In one arrangement, the cathode bufferlayer 225 is an ion-conducting polymer with many of the same propertiesas the ion-conducting polymer in the cathode 220. The cathode bufferlayer 225 provides a region for the proton concentration to transitionfrom the polymer electrolyte membrane 265, which has a highconcentration of protons to the cathode 220, which has a low protonconcentration. Within the cathode buffer layer 225, protons from thepolymer electrolyte membrane 265 encounter anions from the cathode 220,and they neutralize one another. The cathode buffer layer 225 helpsensure that a deleterious number of protons from the polymer electrolytemembrane 265 does not reach the cathode 220 and raise the protonconcentration. If the proton concentration of the cathode 220 is toohigh, COx reduction does not occur. High proton concentration isconsidered to be in the range of approximately 10 to 0.1 molar and lowconcentration is considered to be less than approximately 0.01 molar.

If present, the cathode buffer layer 225 can include a single polymer ormultiple polymers. If the cathode buffer layer 225 includes multiplepolymers, the multiple polymers can be mixed together or can be arrangedin separate, adjacent layers. Examples of materials that can be used forthe cathode buffer layer 225 include, but are not limited to, FumaSepFAA-3, Tokuyama anion exchange membrane material, and polyether-basedpolymers, such as polyethylene oxide (PEO), and blends thereof. Furtherexamples are given above in the discussion of the cathode catalystlayer.

The thickness of the cathode buffer layer can be chosen to be sufficientthat COx reduction activity is high due to the proton concentrationbeing low. This sufficiency can be different for different cathodebuffer layer materials. In some embodiments, the thickness of thecathode buffer layer is between approximately 200 nm and 100 μm, between300 nm and 75 μm, between 500 nm and 50 μm, or any suitable range.

In some embodiments, the cathode buffer layer is less than 50 μm, forexample between 1-25 μm such between 1-5 μm, 5-15 μm, or 10-25 μm. Byusing a cathode buffer layer in this range of thicknesses, the protonconcentration in the cathode can be reduced while maintaining theoverall conductivity of the cell. In some embodiments, an ultra-thinlayer (100 nm-1 μm and in some embodiments, sub-micron) may be used. Andas discussed above, in some embodiments, the MEA does not have a cathodebuffer layer. In some such embodiments, anion-conducting polymer in thecathode catalyst layer is sufficient. The thickness of the cathodebuffer layer may be characterized relative to that of the PEM.

Water and CO₂ formed at the interface of a cathode buffer layer and aPEM can delaminate the MEA where the polymer layers connect. Thedelamination problem can be addressed by employing a cathode bufferlayer having pores. In some embodiments, the cathode buffer layerincludes inert filler particles and associated pores. One possibleexplanation of its effectiveness is that the pores create paths for thegaseous carbon dioxide to escape back to the cathode where it can bereduced.

Materials that are suitable as inert filler particles include, but arenot limited to, TiO₂, silica, PTFE, zirconia, and alumina. In variousarrangements, the size of the inert filler particles is between 5 nm and500 μm, between 10 nm and 100 μm, or any suitable size range. Theparticles may be generally spherical.

If PTFE (or other filler) volume is too high, it will dilute the polymerelectrolyte to the point where ionic conductivity is low. Too muchpolymer electrolyte volume will dilute the PTFE to the point where itdoes not help with porosity. In many embodiments a mass ratio of polymerelectrolyte/PTFE is 0.25 to 2, and more particularly, 0.5 to 1. A volumeratio polymer electrolyte/PTFE (or, more generally, polymerelectrolyte/inert filler) may be 0.25 to 3, 0.5 to 2, 0.75 to 1.5, or1.0 to 1.5.

As noted above, porosity may be introduced by incorporating inert fillerparticles into the cathode buffer layer. In other arrangements, porosityis achieved by using particular processing methods when the layers areformed. One example of such a processing method is laser ablation, wherenano to micro-sized channels are formed in the layers. Another exampleis mechanically puncturing a layer to form channels through it. Anotherexample is appropriately tailoring conditions during ultrasonic spraydeposition of a layer to make it porous.

In one arrangement, the cathode buffer layer has a porosity between0.01% and 95% (e.g., approximately between, by weight, by volume, bymass, etc.). However, in other arrangements, the cathode buffer layercan have any suitable porosity (e.g., between 0.01-95%, 0.1-95%,0.01-75%, 1-95%, 1-90%). In some embodiments, the porosity is 50% orless, e.g., 0.1-50%, 5-50%, 20-50%, 5-40%, 10-40%, 20-40%, or 25%-40%.In some embodiments, the porosity is 20% or below, e.g. 0.1-20%, 1-10%,or 5-10%.

Porosity of the cathode buffer layer or any layer in the MEA may bemeasured as described above with respect to the catalyst layer,including using mass loadings and thicknesses of the components, bymethods such as mercury porosimetry, x-ray diffraction (SAXS or WAXS),and image processing on TEM images to calculate filled space vs. emptyspace. Porosity is measured when the MEA is completely dry as thematerials swell to varying degrees when exposed to water duringoperation. As described further below, the porosity may be determinedusing measured loading and thickness of the layer and known density ofthe material or materials of the layer.

Porosity in layers of the MEA, including the cathode buffer layer, isdescribed further below.

Anode Buffer Layer (MEA Layer Description)

In some CRR reactions, bicarbonate is produced at the cathode 220. Itcan be useful if there is a polymer that blocks bicarbonate transportsomewhere between the cathode 220 and the anode 240, to preventmigration of bicarbonate away from the cathode. It can be thatbicarbonate takes some CO₂ with it as it migrates, which decreases theamount of CO₂ available for reaction at the cathode. In one arrangement,the polymer electrolyte membrane 265 includes a polymer that blocksbicarbonate transport. Examples of such polymers include, but are notlimited to, Nafion® formulations, GORE-SELECT, FumaPEM® (PFSA)(FuMA-Tech GmbH), and Aquivion (PFSA) (Solvay). In another arrangement,there is an anode buffer layer 245 between the polymer electrolytemembrane 265 and the anode 240, which blocks transport of bicarbonate.If the polymer electrolyte membrane is an anion-conductor, or does notblock bicarbonate transport, then an additional anode buffer layer toprevent bicarbonate transport can be useful. Materials that can be usedto block bicarbonate transport include, but are not limited to Nafion®formulations, GORE-SELECT, FumaPEM® (PFSA) (FuMA-Tech GmbH), andAquivion (PFSA) (Solvay). Of course, including a bicarbonate blockingfeature in the ion-exchange layer 260 is not particularly desirable ifthere is no bicarbonate in the CRR.

In another embodiment of the invention, the anode buffer layer 245provides a region for proton concentration to transition between thepolymer electrolyte membrane 265 to the anode 240. The concentration ofprotons in the polymer electrolyte membrane 265 depends both on itscomposition and the ion it is conducting. For example, a Nafion polymerelectrolyte membrane 265 conducting protons has a high protonconcentration. A FumaSep FAA-3 polymer electrolyte membrane 265conducting hydroxide has a low proton concentration. For example, if thedesired proton concentration at the anode 240 is more than 3 orders ofmagnitude different from the polymer electrolyte membrane 265, then ananode buffer layer 245 can be useful to effect the transition from theproton concentration of the polymer electrolyte membrane 265 to thedesired proton concentration of the anode. The anode buffer layer 245can include a single polymer or multiple polymers. If the anode bufferlayer 245 includes multiple polymers, the multiple polymers can be mixedtogether or can be arranged in separate, adjacent layers. Materials thatcan be useful in providing a region for the pH transition include, butare not limited to, Nafion, FumaSep FAA-3, Sustainion®, Tokuyama anionexchange polymer, and polyether-based polymers, such as polyethyleneoxide (PEO), blends thereof, and/or any other suitable materials. Highproton concentration is considered to be in the range of approximately10 to 0.1 molar and low concentration is considered to be less thanapproximately 0.01 molar. Ion-conducting polymers can be placed indifferent classes based on the type(s) of ions they conduct. This hasbeen discussed in more detail above. There are three classes ofion-conducting polymers described in Table 4 above. In one embodiment ofthe invention, at least one of the ion-conducting polymers in thecathode 220, anode 240, polymer electrolyte membrane 265, cathode bufferlayer 225, and anode buffer layer 245 is from a class that is differentfrom at least one of the others.

Layer Porosity

In some embodiments, one or more of the layers of the MEA include poresthat allow gas and liquid transport. These pores are distinct fromion-conduction channels that allow ion conduction. In many polymerelectrolytes (e.g. PFSA), ion conduction occurs through pores lined withstationary charges. The mobile cations hop between the oppositelycharged stationary groups that line the ion conduction channel. Suchchannels may have variable width; for PFSA materials, the ion conductionchannel diameter ranges from narrow areas of approximately 10 Å diameterto wider areas of approximately 40 Å diameter. In anion conductingpolymer materials, the channel diameters may be larger, up to about aminimum width of 60 Å in the narrow areas of the channel.

For efficient ion conduction, the polymer-electrolyte is hydrated, sothe ion conduction channels also contain water. It is common for somewater molecules to move along with the mobile ions in a process termedelectro-osmotic drag; typically 1-5 water molecules per mobile ion aremoved via electro-osmotic drag. The ion-conducting channel structure anddegree of electro-osmotic drag can vary with differentpolymer-electrolytes or ion-conducting materials. While these ionconducting channels allow ions to move along with some water molecules,they do not allow uncharged molecules to move through them efficiently.Nor do they allow bulk water that is not associated with ions to movethrough them. A solid (i.e., non-porous) membrane of a polymerelectrolyte blocks the bulk of CO₂ and products of CO₂ electrolysis frompassing through it. The typical permeability of CO₂, water, and H₂through a wet Nafion 117 PFSA membrane at 30° C. are approximately8.70×10⁶ mol cm cm-2 s-1·Pa-1, 4.2 (mol/cm-s-bar)×10⁹, and 3.6(mol/cm-s-bar)×10¹¹. Permeability depends on temperature, hydration, andnature of the polymer-electrolyte material. In ion conduction channelsthat have variable diameters, uncharged molecules and bulk movement ofliquid/gas may be blocked at least at the narrow parts of the channel.

Pores of larger diameter that the ion conduction channels describedabove allow the passage of bulk liquid and gas, not just ions. Thepolymer electrolyte membrane layer of the MEA typically does not containthis type of pore because the membrane needs to separate reactants andproducts at the cathode from reactants and products at the anode.However, other layers of the MEA may have this type of pore, forexample, the cathode catalyst layer may be porous to allow for reactantCO_(x) to reach the catalyst and for products of CO_(x) reduction tomove out of the catalyst layer, through the gas distribution layer, andout the flow field of the electrolyzer. As used herein, the term porerefer to pores other than the ion conduction channels in an ionomer. Insome embodiments, the pores of anion conducting polymer layer in an MEAhave a minimum cross-sectional dimension of at least 60 Å. In someembodiments, the pores of cation conducting polymer layer in an MEA havea minimum cross-sectional dimension of at least 20 Å. This is todistinguish pores that allow gas/liquid transport from the ionconduction channels described above.

It can be useful if some or all of the following layers are porous: thecathode 220, the cathode buffer layer 225, the anode 240 and the anodebuffer layer 245. In some arrangements, porosity is achieved bycombining inert filler particles with the polymers in these layers.Materials that are suitable as inert filler particles include, but arenot limited to, TiO₂, silica, PTFE, zirconia, and alumina. In variousarrangements, the size of the inert filler particles is between 5 nm and500.mu.m, between 10 nm and 100. mu.m, or any suitable size range. Inother arrangements, porosity is achieved by using particular processingmethods when the layers are formed. One example of such a processingmethod is laser ablation, where nano to micro-sized channels are formedin the layers. Laser ablation can additionally or alternatively achieveporosity in a layer by subsurface ablation. Subsurface ablation can formvoids within a layer, upon focusing the beam at a point within thelayer, and thereby vaporizing the layer material in the vicinity of thepoint. This process can be repeated to form voids throughout the layer,and thereby achieving porosity in the layer. Sublayer by sublayermethods of forming an MEA layer such as ultrasonic spray deposition maybe used to form an MEA layer having a controlled porosity. A dry depositcan lead to faster drying of layers and a more porous final deposit. Oneor more of high substrate temperature, slow deposition rate, highelevation of nozzle from the substrate, and high volatility ofdeposition ink can be used to make the layer more porous. A wet depositcan lead to slower can lead to slower drying of layers, densificationand compaction of several layers for the final deposit. One or more oflow substrate temperature, fast deposition rate, low elevation of spraynozzle from the substrate, and low volatility of the deposition ink canbe used to make the layer less porous. For example, a room temperatureultrasonic spray deposition may result in a relatively dense layer and a50° C. ultrasonic spray deposition may result in a relatively porouslayer.

In some embodiments, the following conditions may be used to form layershaving porosities of at least 1%, e.g., 1-90%, 1-50%, or 1-30% porosity:substrate temperature of at least 40° C.; deposition rate of no morethan 0.8 mL/min, e.g., 0.2-0.8 mL/min; elevation of nozzle of at least50 mm, e.g., 50-75 mm; and solvent volatility of at least 90-100% (e.g.,ethanol).

In some embodiments, the following conditions may be used to form layershaving non-porous layers or layers having porosities of less than 1%:substrate temperature of less than 40° C.; deposition rate of more than0.8 mL/min and up to 10 mL/min; elevation of nozzle of less than 50 mm;and lower solvent volatility of at least 90-100% (e.g., 50-90% volatilesolvent content such as ethanol or 50-100% intermediate volatility ofsolvent such as glycol ethers).

The volume of a void may be determined by the laser power (e.g., higherlaser power corresponds to a greater void volume) but can additionallyor alternatively be determined by the focal size of the beam, or anyother suitable laser parameter. Another example is mechanicallypuncturing a layer to form channels through the layer. The porosity canhave any suitable distribution in the layer (e.g., uniform, anincreasing porosity gradient through the layer, a random porositygradient, a decreasing porosity gradient through the layer, a periodicporosity, etc.).

The porosities (e.g., of the cathode buffer layer, of the anode bufferlayer, of the membrane layer, of the cathode layer, of the anode layer,of other suitable layers, etc.) of the examples described above andother examples and variations preferably have a uniform distribution,but can additionally or alternatively have any suitable distribution(e.g., a randomized distribution, an increasing gradient of pore sizethrough or across the layer, a decreasing gradient of pore size throughor across the layer, etc.). The porosity can be formed by any suitablemechanism, such as inert filler particles (e.g., diamond particles,boron-doped diamond particles, polyvinylidene difluoride/PVDF particles,polytetrafluoroethylene/PTFE particles, etc.) and any other suitablemechanism for forming substantially non-reactive regions within apolymer layer. The inert filler particles can have any suitable size,such as a minimum of about 10 nanometers and a maximum of about 200nanometers, and/or any other suitable dimension or distribution ofdimensions.

As discussed above, the cathode buffer layer preferably has a porositybetween about 1 and 90 percent by volume but can additionally oralternatively have any suitable porosity (including, e.g., no porosity).However, in other arrangements and examples, the cathode buffer layercan have any suitable porosity (e.g., between 0.01-95%, 0.1-95%,0.01-75%, 1-95%, 1-90%, etc.). in some embodiments, the porosity is 20%or below, e.g. 0.1-20%, 1-10%, or 5-10%.

In some embodiments, the cathode buffer layer is porous but at least onelayer between the cathode layer and the anode layer is nonporous. Thiscan prevent the passage of gases and/or bulk liquid between the cathodeand anode layers while still preventing delamination. For example, thenonporous layer can prevent the direct passage of water from the anodeto the cathode.

Porosity of the cathode buffer layer or any layer in the MEA may bemeasured as described above with respect to the catalyst layer,including using mass loadings and thicknesses of the components, bymethods such as mercury porosimetry, x-ray diffraction (SAXS or WAXS),and image processing on TEM images to calculate filled space vs. emptyspace. Porosity is measured when the MEA is completely dry as thematerials swell to varying degrees when exposed to water duringoperation. Porosity can be determined using the known density of thematerial, the actual weight of the layer per given area, and theestimated volume of the layer based on the area and thickness. Theequation is as follows:

${Porosity} = {{100\%} - {\frac{\frac{{layer}{loading}\left( \frac{mg}{{cm}^{2}} \right)}{{density}{of}{material}\left( \frac{mg}{{cm}^{3}} \right)}}{{layer}{thickness}({cm})} \times 100\%}}$

As indicated above, the density of the material is known, and the layerloading and thickness are measured. For example, in a polymerelectrolyte layer with a measured loading of 1.69 mg/cm² made of 42 wt %anion-exchange polymer electrolyte with a density of 1196 mg/cm³ and 58wt % PTFE with a density of 2200 mg/cm³ and a total layer thickness of11.44 microns, the porosity is:

${Porosity} = {{{100\%} - {\frac{\frac{1.69\left( \frac{mg}{{cm}^{2}} \right) \times {0.4}2}{1196\left( \frac{mg}{{cm}^{3}} \right)} + \frac{1.69\left( \frac{mg}{{cm}^{2}} \right) \times {0.5}8}{2200\left( \frac{mg}{{cm}^{3}} \right)}}{0.001144({cm})} \times 100\%}} = {9.1\%}}$

As indicated above, the polymer electrolyte layers may have ionconduction channels that do not easily permit the gas/liquid transport.In the calculation above, these ion conduction channels are considerednon-porous; that is, the density of the non-porous material above (42 wt% anion-exchange polymer electrolyte) includes the ion conductionchannels and is defined by the calculation to be non-porous.

In another example, an ion conductive layer without filler is porous.Porosity may be introduced by appropriate deposition conditions, forexample. The measured loading of the porous polymer electrolyte layer is2.1 g/cm² and the thickness is 19 micrometers. The known density of thepolymer electrolyte with ion-conducting channels but without pores is1196 g/cm³. The porosity is then calculated as:

${Porosity} = {{{100\%} - {\frac{\frac{2.1\left( \frac{mg}{{cm}^{2}} \right)}{1196\left( \frac{mg}{{cm}^{3}} \right)}}{{0.0}019({cm})} \times 100\%}} = {3.2\%}}$

MEA Fabrication

MEAs for CO_(x) reduction may be fabricated using a variety oftechniques. In various embodiments, MEAs fabrication employs multiplesteps. Small differences in the parameters of the fabrication processcan make a large difference in performance.

In certain embodiments, MEA fabrication employs a polymer-electrolytemembrane (e.g., a Nafion PEM) layer and depositing or otherwise formingan anion-exchange polymer electrolyte layer and cathode catalyst layeron the cathode side of the membrane and depositing or otherwise formingan anode catalyst layer on the anode side of the membrane. An alternateroute is to fabricate the catalyst layers on to porous gas diffusionlayers (e.g., carbon for the cathode or titanium for the anode) andsandwich the membrane (which may include the anion-exchange layer)between catalyst containing porous layers. In certain embodiments,catalyst layers are fabricated by making an ink of the solid catalystand support particles and polymer electrolyte dispersed in a solvent.The ink may be applied by a variety of methods to the polymerelectrolyte membrane or GDL. The solvent subsequently evaporates leavingbehind a porous solid catalyst layer.

Imaging methods may be used to characterize the thickness, uniformity,and surface roughness. The thickness should be consistent andcontrollable, and the uniformity smooth and as defect free as possible.

Various techniques may be employed to form the individual layers of theMEA. Generally, these techniques form the layer on a substrate such as aPEM layer or GDL as mentioned herein. Examples of such techniquesinclude ultrasonic spray deposition, doctor blade application, gravure,screen printing, slot die coating, and decal transfer.

Catalyst inks using anion-exchange polymers are not well studied(particularly for certain polymers) and do not have the same solutionstructure as typical Nafion-based inks used in fuel cells andelectrolyzers. The formulation and steps needed for form a welldispersed and stable catalyst ink were not known. It is believed thatNafion forms micell-like structures that allow relatively easysuspension in aqueous media. Other ion-conducting polymers andparticularly some anion-conducting polymers do not form such structuresand therefore are more difficult to provide in suspensions.

In certain embodiments, a catalyst layer ink is prepared by mixing metalor metal supported on carbon catalyst with ion-conducting polymer (e.g.,an anion-conducting polymer) and dispersing in solvent (alcohol, etc.)by sonicating.

As indicated, certain fabrication techniques utilize doctor bladeapplication, screen printing, decal transfer, electrospinning, etc.Roll-to-roll techniques such as gravure or microgravure or slot diecoating may be used for high throughput processing.

In some embodiments, the cathode side of the MEAS is fabricated by firstdepositing a layer of anion-exchange polymer-electrolyte on top of acation-exchange polymer electrolyte membrane. Then a second layer ofcathode catalyst is applied on top of the anion-exchange layer. Thisprocess produces a catalyst coated membrane. Gas diffusion electrodesmay be prepared by depositing the catalyst onto a gas diffusion layer.The anion exchange layer can be deposited onto the catalyst layer or themembrane. The layers can then be pressed together inside theelectrolysis cell to make a functioning device. Many methods, includingdoctor blade, gravure or micro gravure, slot die, decal transfer, screenprinting, ultrasonic spray deposition and others can be used tofabricate the anion-exchange polymer layer and the cathode catalystlayer. A more detailed description of MEA cathode fabrication usingultrasonic spray deposition follows:

The cathode side of the MEA is fabricated by first forming a solution ofpolymer-electrolyte (approximately 1-25 wt %) in a suitable solvent,such as ethanol, n-propanol, isopropanol, or other high vapor pressureand/or low boiling point solvent that will evaporate on a reasonabletimescale during fabrication. Mixtures of solvents with one or morehigher boiling point components can be used. The polymer electrolytesolution is pushed through an ultrasonic spray deposition nozzle at adesired flow rate. The ultrasonic spray deposition nozzle is held at thedesired frequency to disperse the polymer-electrolyte solution intosmall droplets that are then pushed by an air stream onto apolymer-electrolyte membrane substrate. The polymer-electrolyte membranemay be treated with heat, solvent, or other means before deposition. Thesmall droplets of polymer-electrolyte solution land onpolymer-electrolyte membrane substrate where the solvent evaporates andleaves behind the entrained polymer-electrolyte. The ultrasonic spraydeposition nozzle moves back and forth across the substrate multipletimes with the desired pattern with the desired speed to build up apolymer-electrolyte layer on top of the membrane substrate until thedesired thickness is reached. This process is then repeated using asolution of catalyst particles, anion-exchange polymer electrolyteand/or other additives, and a suitable solvent or mixture of solvents;this solution is termed the catalyst ink. The catalyst ink is depositedvia ultrasonic spray deposition using the same or different fabricationparameters to form the cathode catalyst layer on top of theanion-exchange polymer layer on the cathode side of the MEA.

MEA Scale Up

As indicated, certain applications of MEAs for CO_(x) reduction mayrequire relatively large formats. For example, some MEAs have activesurface areas (excluding pores) of at least about 500 cm². And in someother embodiments, MEAs have even larger active surface areas (excludingpores), or e.g., at least about 650 cm² or 1500 cm².

To make MEAs with such large active surface areas, an appropriatemanufacturing process must be chosen, i.e., a process that can supportlarge volumes of catalyst ink and large surface areas to which thecatalyst ink is applied. Scaling up the catalyst ink requires particularmethods of dispersing the catalyst particles to ensure good dispersionin large volumes. The ink may be set at a target dispersity, whichdynamic light scattering (DLS) can be used to characterize. The inkshould be stable within the time range of the layer deposition.

Additionally, humidity and temperature should be tightly controlled.Evaporation rates and processes impact the resulting deposition, socontrolling these things within a 1-2 degree temperature window, androughly 5% RH range is useful.

For ultrasonic spray deposition, thin lines of catalyst ink are laiddown by a moving ultrasonic nozzle. The nozzle movement speed and inkflow rate may need to be increased for larger area MEAs. The flow rateand move speed are at least be doubled going from 25 cm² to a 650 cm²scale MEA. Water in the solvent is important and adding more water intothe ink helps the stacking of droplets be smoother. For example, about20% water in the formulation may be used for a 650 cm² MEA.

Catalyst inks are generally relatively less stable, so in certainembodiments, the MEA fabrication time is designed to be relativelyshort, even when active area is larger. As an example, for a 650 cm²spray, a deposition time of about 2 hours for the ionomer layer and 1hour for the catalyst layer may be used. This is relatively fast forsuch a larger area and can be achieved using the fast flow rate and movespeed.

MEA Scale Up Examples

Below are examples of scaling up MEA fabrication. Examples are providedfor scaling from 25 cm² to 650 cm².

Solvent mixture adjusted (water to alcohol ratio): depending on the sizeof the spray scale, solvent adjustment from 10% water to 20% watersignificantly helps with surface uniformity of the surface

Deposition parameters:

-   -   for the ionomer layer: flow rate is increased from 0.4 mL/min to        0.8 mL/min and move speed is changed from 50 mm/s to 100 mm/s    -   for the catalyst layer: flow rate is increased from 0.25 mL/min        to 0.5 mL/min and move speed is changed from 80 mm/s to 160 mm/s

Morphology and thickness: Thickness can be matched from looking at thethickness of fabricated layers in SEM images. Adjustments can be made oncharacterization data to match the thickness. The morphology iscontrolled with parameters such as water content and fabrication.

For further scale up, e.g., to 1500 cm², flow rate and move speed may befurther increased, e.g., with ranges being from 0.25-2 mL/min and 30-200mm/s.

The speed of deposition can be further increased, e.g., to 5-8 mL/min or5-15 mL/min, by increasing the weight of solids in the solution. In someembodiments, the solution may be greater than 5 wt. %, greater than 10wt. %, greater than 20 wt. %, or greater than 30 wt. %.

MEA Post Treatments

After the MEA is fabricated, additional treatments may be used toincrease performance. Examples the types of performance improvementinclude lifetime and voltage. These improvements may be manifest in MEAsthat have structural modifications resulting from the treatmentsincluding better adhesion between layers.

MEA Post Treatment Examples

Hot pressing: heating the MEA under pressure to bond the layerstogether. Hot pressing is a step sometimes used in MEA fabrication wherethe MEA including the membrane and catalyst layers and sometimes GDLsare compressed together for a period of time at a desired temperature.Hot pressing is used to decrease the interfacial resistance and increaseadhesion between layers and can help ‘melt’ layers together to preventdelamination. Example times, temperatures, and pressures are givenbelow:

-   -   Time: about 2 min to 10 min (MEA only); 1.5 min to 2 min        (MEA+gas distribution layer (GDL)); the “MEA+GDL” may be pressed        at least twice to form a stable assembly    -   Temperature: about 100° C. to 195° C.;    -   Pressure: between 28 psi and 2900 psi. In one example, between        about 300 psi and 600 psi may be used for a 3×3 inch ½ MEA but        the MEA can tolerate about 2500 psi without GDL;

The temperature of the hot press is typically selected so that it isabove the glass transition temperature of the polymer electrolyte, butbelow the temperature where any materials of the MEA become structurallyor chemically damaged. The glass transition temperature is thetemperature above which the polymer-electrolyte becomes soft, which mayallow for the polymer-electrolyte at layer interfaces to deform and forma better contact with lower ionic transport resistance and betteradhesion.

Hydration: soaking the MEA in water or aqueous solutions to wet thepolymer-electrolytes prior to cell assembly

Boil Nafion or other polymer electrolyte MEA. This permanently changesthe macrostructure of the polymer electrolyte and increases the amountof water in the polymer matrix. This increases ionic conductivity, butalso increases water transport number.

Heat to dry. This permanently decrease water content and can reduce theamount of water transported through the polymer electrolyte duringoperation. Example times and temperatures for heating various MEAs arebelow.

Time Temperature MEA (Hour) (° C.) Nafion 115 25 cm² ½ MEA 24 10-30Nafion 115 100 cm² ½ MEA 48 10-30 Nafion 117 25,100 cm² ½ MEA 24 10-30Nafion 212 ½ MEA 24 10-30 Nafion 211 ½ MEA 24 10-30 ½ MEA refers to thepolymer-electrolyte membrane coated with the anode catalyst layer on oneside.

Stabilized Interface Between MEA Layers

Water and CO₂ formed at the interface of an anion-conducting layer(e.g., a cathode buffer layer) and a cation-conducting membrane (e.g., aPEM) can cause the two layers to separate or delaminate where thepolymer layers connect. The reaction at the bipolar interface isdepicted in FIGS. 3 and 9 .

In addition, it is desirable for the CO₂ to return to the cathode of thecell where it can be reduced instead of lost to the anode, so a pathway(e.g., pores) in an anion-exchange layer (e.g., a cathode buffer layerand/or cathode layer) provides both a way to remove water and CO₂ fromthe interface to prevent delamination and return CO₂ to the cathodewhere it can react.

FIG. 9 is similar to FIG. 3 , but it includes additional informationrelevant to mass transport and generation of CO₂ and water at a bipolarinterface. For example, it shows hydroxide and CO₂ reacting on thecathode side to produce bicarbonate ions, which move toward the bipolarinterface 913. On the anode side, hydrogen ions produced by wateroxidation move toward bipolar interface 913, where they react with thebicarbonate ions to produce water and CO₂, both of which should beallowed to escape without damaging the bipolar layers.

Also depicted in FIG. 9 are water transport paths including (a)electroosmotic drag with anions from the cathode to interface 9, (b)electroosmotic drag with cations from the anode to interface 913, and(c) diffusion. Water evaporates at the anode and cathode.

Various MEA designs contain features that resist delamination andoptionally provide a pathway for the reaction products to leave theinterface area. In some embodiments, the bipolar interface is flat. Butin some designs, the interface is provided with a composition gradientand/or interlocking structures. These are described further below withreference to FIGS. 10 a, 10 b, 10 c , and 10 d, which illustrate bipolarinterfaces of MEA designs configured to resist delamination.

Engineering the interface can be used to reduce undesired co-ion leakagethrough the anion exchange membrane (AEM) and cation exchange membrane(CEM) and improving the mechanical stability of bipolar membrane withbetter adhesion. Chemical and physical modifications to the interfacecan be used to achieve these two goals. As described further below, theAEM and CEM layers can be chemically bonded through multiplecross-linking pathways: side chain, backbone, backbone-to-side-chain,and triple cross-linking. In some embodiments, the AEM and CEM layersinterpenetrate. This can include one or more of a gradient ofanion-exchange and cation-exchange polymers, a mixture of anion-exchangeand cation-exchange polymers, and/or protrusions of at least one polymerextending into the other.

There are also different ways to physically modify the interface.Hot-pressing the AEM and CEM close to their respective glass transitiontemperature can increase the adhesion between the AEM and CEM. In someembodiments, adhesion is improved by increasing the interfacial surfacearea through electrospinning anion and cation exchange layers. In suchembodiments, the anion and cation exchange ionomers to have similarswelling properties to avoid delamination. Adding a small concentrationof a third polymer (e.g. PTFE) to the intertwined ionomers could alsofacilitate water removal from the interface. The surface of both the CEMand AEM can be intentionally roughened through plasma surface treatment,etching, or hot-pressing with a woven or patterned fabric. One or moreof these techniques may be used to increase contact between the AEM andCEM.

In some embodiments, the interface includes a gradient. A gradient maybe formed, for example, by using two nozzles during spray deposition andadding anion-exchange polymer with the relative amounts of the polymersvaried during deposition of the cation-exchange layer. Similarly,cation-exchange polymer may be added during deposition of theanion-exchange layer. Referring for example to FIG. 9 , a gradient mayextend through substantially all or a portion of the anion-exchangeregion and cation-exchange region, such that the anion-exchange regionhas predominantly anion-exchange polymer adjacent to the cathode withthe relative amount of cation-exchange polymer increasing moving fromthe cathode toward the interface 913. Similarly, the cathode-exchangeregion has a predominantly cation-exchange polymer adjacent the anodecathode with the relative amount of anion-exchange polymer increasingmoving from the anode toward the interface 913. In some embodiments,there are a pure anion-exchange and pure cation-exchange regions with agradient between the two.

In some embodiments, the layers of the bipolar membrane are meltedtogether. This may be accomplished by choosing an appropriate solvent.For example, Nafion is at least slightly soluble in a water/ethanolmixture. By using that mixture (or another solvent in which thecation-conducting polymer is soluble) as a solvent for theanion-conducting polymer can result in Nafion or other cation-conductingpolymer at least slightly dissolvent and melting into the interface. Insome embodiments, this results in a thin gradient, e.g., one thatextends 0.5-10% into the anion-conducting polymer layer thickness.

In some embodiments, the interface includes a mixture of the polymers.FIG. 10 a illustrates a bipolar interface 1013 in which acation-conducting polymer 1021 and an anion-conducting polymer 1019 aremixed. In the example of FIG. 10 a , a portion of an anion-conductingpolymer layer 1009 and a portion of a cation-conducting polymer layer1011 are shown. The anion-conducting polymer layer 1009 may be a pureanion-conducting polymer and the cation-conducting polymer layer 1011may be pure cation exchange polymer. The cation-conducting polymer 1021may be the same or different cation-conducting polymer as in thecation-conducting polymer layer 1011. The anion-conducting polymer 1019may be the same or different anion-conducting polymer as in theanion-conducting polymer layer 1009.

In some embodiments, the interface includes a third material thatphysically reinforces the interface. For example, FIG. 10 b shows anexample of a material 1030 that straddles interface 1013. That is, thematerial 1030 partially resides in an anion-conducting polymer layer1009 and a cation-conducting polymer layer 1011. Because of this,material 1030 may bind the two layers in a manner that resistsdelamination. In one example, the material 1030 is an inert material,such as PTFE, polyvinylidene difluoride (PVDF), a charged colloidalsphere such as a surface-modified metal hydroxide sphere such as Al(OH)₃with trimethylaluminum (TMA). The inert material may be in the form of aweb or mesh with gaps that can be filled by the ionomers. Such aninterface may be fabricated, for example, by casting or otherwiseapplying the cation-conducting polymer and the anion-conducting polymeron opposite sides of a PTFE mesh or similar structure, followed by hotpressing.

FIG. 10 c illustrates a bipolar interface 1013 having protrusions 1040of the cation-conducting polymer extending from the cation-conductingpolymer layer 1011 into the anion-conducting polymer layer 1009. Theseprotrusions may mechanically strengthen interface 1013 so that it doesnot delaminate when CO₂ and water are produced at the interface. In someembodiments, protrusions extend from anion-conducting polymer layer 1009into cation-conducting polymer layer 1011. In certain embodiments,protrusions extend both directions. Example dimensions are 10 μm-1 mm inthe in-plane dimension, though smaller dimensions (e.g., 500 nm-1 μm)are possible. The out-of-plane dimension may be for example, 10-75% or10-50% of the total thickness of the anion exchange layer. Theprotrusions may be fabricated for example by any appropriate techniquesuch as lithographic techniques or by spraying the polymer into apatterned mesh that is then removed. Surface roughening techniques mayalso be used to create protrusions. In some embodiments, protrusions maybe formed from a different material, e.g., a non-ion-conducting polymer,a ceramic, or a metal to help interlock the polymer layers andmechanically strengthen the interface.

FIG. 10 d illustrates a bipolar interface 1013 having a third material1050 disposed between or mixed one or more of the cation-conductingpolymer layer 1011 into the anion-conducting polymer layer 1009. In someembodiments, for example, the third material 1050 can be an additive asdiscussed further below. In some embodiments, the third material 1050can be a blend of anion-conducting and cation-conducting ionomers at theinterface. For example, it can be a mixture of Nafion 5 wt % ionomer andOrion 2 wt % mTPN1. In some embodiments, the third material may includeion acceptors and donors, either mixed together or provided as distinctlayers.

In some embodiments, the interface includes additives to facilitateacid-base reactions and prevent delamination. In some embodiments, theadditives may facilitate spreading out the acid base recombination alarger volume instead of just at a 2D interface of the anion conductingpolymer and cation conducting polymer. This spreads out water and CO₂formation, heat generation, and may lower the resistance of the membraneby decreasing the barrier to the acid-base reaction. These effects canbe advantageous in helping avoid build-up of products, heat, andlowering resistive losses in the MEA leading to a lower cell voltage.Further, it helps avoid degrading materials at the interface due to heatand gas production.

Examples of additives that facilitate acid-base reactions includemolecules that are both proton and anion acceptors, such as hydroxidecontaining ionic liquids with 1-butyl-3-methylimidazolium hydroxidebeing a specific example. Other ionic liquids may also be used,including those having one of the following ionic groups:N,N,N,N-tetraalkylammonium (e.g., N,N,N,N-tetramethylammonium,N,N-dimethyl-N,N-dipropylammonium, or N-methyl-N,N,N-tri-C₁₋₁₂alkylammonium), N,N,N-trialkylammonium-1-yl (e.g.,N,N,N-trimethylammonium-1-yl, N-methyl-N,N-dipropylammonium-1-yl, orN,N,N-tri-C₁₋₁₂ alkylammonium-1-yl),N,N,N-trialkyl-N-alkoxyalkylammonium (e.g.,N,N,N-trimethyl-N-alkoxyalkylammonium,N-methyl-N,N-diethyl-N-methoxyethylammonium, or N,N,N-tri-C₁₋₁₂alkyl-N—C₁₋₆ alkoxy-C₁₋₆ alkylammonium),N,N-dialkyl-N-alkoxyalkylammonium-1-yl (e.g.,N,N-dimethyl-N-alkoxyalkylammonium-1-yl or N,N-di-C₁₋₁₂ alkyl-N—C₁₋₆alkoxy-C₁₋₆ alkylammonium-1-yl), N,N-dialkylpyrrolidinium (e.g.,N,N-dimethylpyrrolidinium, N-methyl-N-ethylpyrrolidinium, orN-methyl-N—C₁₋₁₂ alkylpyrrolidinium), N-alkylpyrrolidinium-1-yl (e.g.,N-methylpyrrolidinium-1-yl or N—C₁₋₁₂ alkylpyrrolidinium-1-yl),N,N-dialkylpiperidinium (e.g., N,N-dimethylpiperidinium,N-methyl-N-ethylpiperidinium, or N-methyl-N—C₁₋₁₂ alkylpiperidinium),N-alkylpiperidinium-1-yl (e.g., N-methylpiperidinium-1-yl or N—C₁₋₁₂alkylpiperidinium-1-yl), N,N,4-trialkylpiperidinium (e.g.,N,N,4-trimethylpiperidinium, N,4-dimethyl-N-ethylpiperidinium, orN-methyl-N,4-di-C₁₋₁₂ alkylpiperidinium), N,4-dialkylpiperidinium-1-yl(e.g., N,4-dimethylpiperidinium-1-yl or N,4-di-C₁₋₁₂alkylpiperidinium-1-yl), N,N,3,5-tetraalkylpiperidinium (e.g.,N,N,3,5-tetramethylpiperidinium, N,3,5-trimethyl-N-ethylpiperidinium, orN-methyl-N, 3,5-tri-C₁₋₁₂ alkylpiperidinium),N,3,5-trialkylpiperidinium-1-yl (e.g., N,3,5-trimethylpiperidinium-1-ylor N,3,5-tri-C₁₋₁₂ alkylpiperidinium-1-yl),N,N,2,6-tetraalkylpiperidinium (e.g., N,N,2,6-tetramethylpiperidinium,N,2,6-trimethyl-N-ethylpiperidinium, or N-methyl-N, 2,6-tri-C₁₋₁₂alkylpiperidinium), N,2,6-trialkylpiperidinium-1-yl (e.g.,N,2,6-trimethylpiperidinium-1-yl or N,2,6-tri-C₁₋₁₂alkylpiperidinium-1-yl), N,N-dialkylazepanium (e.g.,N,N-dimethylazepanium, N-methyl-N-ethylazepanium, or N-methyl-N—C₁₋₁₂alkylazepanium), N-alkylazepanium-1-yl (e.g., N-methylazepanium-1-yl orN—C₁₋₁₂ alkylazepanium-1-yl), N,N-dialkylmorpholinium (e.g.,N,N-dimethylmorpholinium, N-methyl-N-ethylmorpholinium, orN-methyl-N—C₁₋₁₂ alkylmorpholinium), N-alkylmorpholinium-4-yl (e.g.,N-methylmorpholinium-4-yl or N—C₁₋₁₂ alkylmorpholinium-4-yl),N1,N3-dialkylimidazolium (e.g., N1,N3-dimethylimidazolium,N1-ethyl-N3-methylimidazolium, or N1-C₁₋₁₂ alkyl-N3-methyl-imidazolium),N3-alkylimidazolium-1-yl (e.g., N3-methylpiperidinium-1-yl or N3-C₁₋₁₂alkylpiperidinium-1-yl), 1-alkyl-1-azabicyclo[2.2.2]octane (e.g.,1-methyl-1-azabicyclo[2.2.2]octane or 1-C₁₋₁₂alkyl-1-azabicyclo[2.2.2]octane), or 1-azoniabicyclo[2.2.2]octan-1-yl,in which each of these can be optionally substituted (e.g., substitutedon a ring with one or more alkyl and/or substituted on an alkyl with oneor more heteroatoms).

In some embodiments, an ionomer different from that of theanion-conductive polymer layer and the cation-conductive polymer layermay be used. For example, a relatively high conductivity anion-exchangematerial such as Sustainion may be used. Such anion-exchange materialmay not be selective enough to use as a cathode buffer layer but can beused at the interface.

In particular examples, an ionomer may be used at the interface that hasa higher ion exchange capacity than at least one of the ionomers of thebipolar membrane. Such an ionomer may not be suitable for the layers ofthe bipolar membrane, for example, due to propensity to swelling or lackof stability, but can be added at the interface. In particular examples,an ionomer that improves adhesion and physical contact may be used. Apolymer at the interface that goes into both layers may be used toimprove adhesion. An ionomer at the interface can itself have multiplesublayers. In one example, a third ionomer may have center region havinghigher void space disposed between denser regions.

In some embodiments, an ionomer used at the interface is an anionexchange ionomer that is different from the anion-conducting polymer ofthe anion conducting polymer layer and may be referred to as aninterface AEM to distinguish it from the bulk AEM of the anionconducting polymer layer. In some such embodiments, the interface AEMhas lower water uptake than the anion-conducting polymer layer to matchthe water uptake of PFSA or other cation-conducting polymer. This canhelp prevent delamination at the interface while maintaining a higherion exchange capacity (IEC). Both the higher IEC and lower water uptakeof the interface ionomer may help minimize cation crossover from theanode side. Lower water uptake can result from smaller ion conductionchannels in the interface ionomer than in the anion-conducting polymerof the bipolar membrane. Higher IEC can result from a higherconcentration of cation functional groups on the interface ionomer. Oneor both of these characteristics may be present in the interface ionomerand can restrict cations from the cathode.

In particular embodiments, when forward bias is applied across a bipolarmembrane, ion recombination occurs at the interface to form productssuch as water. An interfacial layer should be mechanically robust duringion recombination (i.e., exhibit good adhesion between AEM and CEM ofthe bipolar membrane) while minimizing undesired co-ion leakage throughthe AEM and CEM. In some embodiments, an interface AEM has a thicknessof 0.1%-10% of the bulk AEM thickness, with examples of bulk AEMthickness being between 5-80 μm. The interface AEM1 and 90 percent byvolume may be kept relatively low to avoid additional ohmic resistancesacross the bipolar membrane. The water uptake of the interface AEM canbe between 0%-25% to circumvent membrane delamination due to a mismatchof swelling properties between the adjacent AEM and CEM. In someinterface AEM can have an ion exchange capacity (IEC) in the range of2.5-3.0 mmol/g. In some such embodiments, the IEC of the bulk AEM islower than that of the interface AEM and may be 1.5-2.5 mmol/g. A highdensity of positively-charged functional groups (i.e. high IEC) at theinterface serves to electrostatically repel undesired co-ion (e.g. H+ orK+) transport to the bulk AEM via the Donnan exclusion effect.

Additional examples of materials that may be present at the interfaceinclude block copolymers having different charged groups (e.g., bothcation and anion stationary charge groups), cation-and-anion conductingpolymers, resin material, ion donors such as oxides including grapheneoxide, catalysts for acid/base recombination, catalysts that react H₂and O₂ diffusing from the anode and cathode, water splitting catalysts,CO₂ absorbing material, and H₂ absorbing material.

In some embodiments, the anion-conducting polymer and thecation-conducting polymer of the bipolar membrane have the samebackbone, with different stationary charge groups. As an example, Orionionomers may be used with different stationary charge groups. Theionomers are more compatible and less apt to delaminate.

In the examples above, the interface 1013 may be a three-dimensionalvolume having thickness that is between 1% and 90% of the overallthickness of the bipolar membrane, or between 5% and 90%, or between 10%and 80%, or between 20% and 70%, or between 30% and 60% of the overallthickness of the bipolar membrane. In some embodiments, it less thanhalf the overall thickness, including between 1% and 45%, 5% and 45%, 5%and 40%, or 5% and 30%.

Any of the bipolar interfaces described above may be hot pressed.Particularly between the anion-exchange and cation exchange membranelayers, hot pressing can soften the polymer electrolytes and allow themto meld together.

In some embodiments, the bipolar AEM/PEM interface includes a relativelysmooth PEM layer in contact with a rougher AEM layer. For example, a PEMarithmetic mean height (S_(a)) in such embodiments can range from near 0to 0.2 μm. The AEM layer in contact with the PEM layer can have higherroughness, and in some embodiments have an S_(a) in the 0.2 to 0.5 μmrange, in the 0.4 to 1.5 μm range, or in the 0.6 to 1 μm range. Theroughness of the AEM with the PEM can create a discontinuous interface.The S_(a) of the AEM layer in contact with the PEM can be lowered tonear 0 to 0.2 μm or near 0 to 1 μm through changes to fabricationparameters, such as treatment with solvent that partially dissolves thepolymer electrolyte before evaporating to leave behind a smoothersurface or hot pressing. The AEM layer may be substantially continuousand non-porous, or it may contain pores with typical porosity ranges canbe 0.1 to 90%, 1-20%, and 5-15% that allow for gas and/or watermovement.

In another embodiment, the surface of the PEM membrane may be roughenedto an S_(a) of 5 to 10 μm, 1 to 5 μm, 0.2 to 1 μm, or 0.4 to 0.6 μm. Insome such embodiments, the AEM layer in contact with the PEM membranemay have S_(a) near 0 to 1 μm or near 0 to 0.2 μm, or it may be rougher,with a S_(a) in the 2 to 5 μm range, in the 0.4 to 1.5 μm range, or inthe 0.6 to 1.0 μm range. The AEM may be substantially continuous andnon-porous, or it may contain pores with typical porosity ranges of 0.1to 90%, 1-20%, and 5-15% that allow for gas and/or water movement.

In some embodiments, a cross-linker may be added to covalentlycross-link the two polymers of the bipolar membrane. A crosslinker canbe used at an interface between ion-conducting polymer layers. Eachlayer can include one or more polymers, in which each polymer can becharacterized by a backbone and a side chain attached to the backbone. Acrosslinking reaction can occur at the interface, as well as between acrosslinker and (i) two or more side chains, (ii) two or more backbones,or (iii) a combination of two or more side chain(s) and backbone(s).

The crosslinker can be bivalent, trivalent, tetravalent, or other highervalency. In this way, the crosslinker can react with any number ofreactive groups present at the interface within the cation-conducting,anion-conducting, or mixed cation-and-anion-conducting polymer layers.In some embodiments, the crosslinker includes:

in which Ak is an optionally substituted aliphatic, alkylene,cycloaliphatic, or cycloalkylene; Ar is an optionally substitutedaromatic, arylene, heteroaromatic, or heteroarylene; L is a linkingmoiety (e.g., any herein); L3 is an integer that is 2 or more; and X ishalo, hydroxyl, optionally substituted amino (e.g., NR^(N1)R^(N2), inwhich each of R^(N1) and R^(N2) is, independently, H or optionallysubstituted alkyl), carboxyl, acyl halide (e.g., —C(O)—R, in which R ishalo), carboxyaldehyde (e.g., —C(O)H), or optionally substituted alkyl.

Non-limiting crosslinkers can include terephthalaldehyde,glutaraldehyde, ortho-xylene, para-xylene, meta-xylene, or a multivalentamine, such as diamine, triamine, tetraamine, pentaamine, etc.,including 1,6-diaminohexane (hexanediamine, DHA),N,N′-dimethyl-1,6-hexanediamine, N,N,N′,N′-tetramethyl-1,6-hexanediamine(TMHDA), 1,3-diaminopropane, N,N′-dimethyl-1,3-propanediamine,N,N,N′,N′-tetramethyl-1,3-propanediamine, 1,4-diaminobutane,N,N′-dimethyl-1,4-butanediamine,N,N,N′,N′-tetramethyl-1,4-butanediamine, 1,8-diaminooctane,N,N′-dimethyl-1,8-octanediamine,N,N,N′,N′-tetramethyl-1,8-octanediamine, propane-1,2,3-triamine,[1,1′:3′,1″-terphenyl]-4,4″,5′-triamine, 1,3,5-triazine-2,4,6-triamine(melamine), and others.

In some embodiments, a crosslinker is used for crosslinking between sidechain groups of the first and second polymer layers. The side chaingroup can include a reactive group that is either present within thematerial or installed in any useful manner.

For instance, if a polymer layer includes an ionic or ionizable sidechain group (e.g., —SO₂OH, —CO₂H, etc.), then this group can beconverted to provide a reactive group (e.g., a halo or a leaving group).In one non-limiting embodiment, as seen in step (i) of Scheme I above,the first polymer (1) includes an ionic side chain group (—SO₂OH), whichis converted to a reactive group (—SO₂Cl in (2)) by use of thionylchloride. The second polymer, in turn, can also include a reactive sidechain group(s) (e.g., a halide, such as —Br, haloalkyl, or anotherleaving group), as in (4). By using a crosslinker, crosslinks are formedbetween reactive groups. As seen in step (ii) of Scheme I above, thereactive groups in the first polymer (2) and the second polymer (4) arereacted with a crosslinker that is a multivalent amine (3). In this way,crosslinks (5) are formed at the interface and between the side chaingroups. In one instance, the polymer layers can be crosslinked based onthe formation of two or more covalent bonds (e.g., an N—S covalent bond,N—C covalent bond, or C—C covalent bond).

In other embodiments, a crosslinker forms crosslinks between backbone(s)of the first and second polymer layers.

In one non-limiting embodiment, as seen in Scheme II above, the firstpolymer (6) and the second polymer (8) include an aryl backbone. Then, acrosslinker is used to react with backbone groups. If the crosslinker isa multivalent hydroxyalkyl as in (7), then the two polymer layers can becrosslinked by way of a be an acid-catalyzed in the presence of a protonsource, such as an organic acid (e.g., trifluoromethanesulfonic acid,sulfuric acid, methanesulfonic acid, trifluoroacetic acid,p-toluenesulfonic acid, etc.). In one instance, the hydroxyalkylcrosslinker can be a tertiary alcohol, which is protonated by a Bronstedacid, lose water as a byproduct, and form a tertiary carbocationintermediate. This intermediate, in turn, can readily react with the πelectrons of the aromatic backbone based on electrophilic substitution.In this method, aromatic backbone from the polymer layers could begrafted with a crosslinker to from a high-density polymer matrix.

In yet other embodiments, a crosslinker is used for crosslinking betweenside chain group(s) of the first polymer layer and backbone(s) of thesecond polymer layer. For instance, the side chain group can beconverted into a nucleophile, and the backbone can include anelectrophile.

In one non-limiting embodiment, as seen in Scheme III above, the firstpolymer (1) includes an ionic side chain group (—SO₂OH), which isconverted to a reactive group (—SO₂Cl in (2)) by use of thionyl chlorideand then aminated to provide a reactive nucleophilic group (e.g., asulfonamide, such as —SO₂NR^(N1)R^(N2) in (10)). The second polymer (8)can include an aryl backbone, which can be reacted with a multivalentcrosslinker. For instance, the crosslinker (11) can be ahydroxyhaloalkyl, which can react by way of an acid-catalyzedFriedel-Crafts alkylation reaction to provide an alkylated polymer (12).Finally, the first polymer (10) having a nucleophilic group can bereacted with the second polymer (12) having an electrophilic group toprovide a crosslinked polymer (13). Alternative chemistries, reactivegroups, electrophiles, and nucleophiles can be used to provide reactivepairs in the first and second polymers that can react at the interface.

Thickness of Layers of MEA

In certain embodiments, a polymer electrolyte membrane and an adjoiningcathode buffer layer or other anion-conducting polymer layer may haverelative thickness that facilitate the fabrication and/or operatingperformance of an MEA.

FIG. 11 depicts an example of partial layers of an MEA, which partiallayers includes an anion-conducting polymer layer (AEM) 1103, which maybe a cathode buffer layer, and a polymer electrolyte membrane (PEM)1105, which may be cation-conducting polymer layer (e.g., a protonexchange polymer layer) or an anion-conducting polymer layer. In thisexample, the PEM 1105 is relatively thicker than the anion-conductingpolymer layer 1103. For example, the PEM 1105 may be 120 micrometerscompared with about 10-30 or 10-20 micrometers thick for the AEM 1103.The PEM 1105 may provide mechanical stability to the AEM 1103.

In some cases, anion-conducting polymers such as those used inanion-conducting polymer layer 1103 are substantially less conductivethan cation-conducting polymers such as those used in PEM 1105.Therefore, to provide the benefits of a cathode buffer layer (e.g.,anion-conducting polymer layer 1103) without substantially increasingthe overall resistance of the MEA, a relatively thin cathode buffer isused. However, when a cathode buffer layer becomes too thin, it becomesdifficult to handle during fabrication of the MEA and in other contexts.Therefore, in certain embodiments, a thin cathode buffer layer isfabricated on top of a relatively thicker PEM layer such as acation-conducting polymer layer. The anion-conducting polymer layer maybe fabricated on the PEM layer using, for example, any of thefabrication techniques described elsewhere herein.

In various embodiments, the polymer electrolyte membrane layer isbetween about 20 and 200 micrometers thick. In some embodiments, thepolymer electrolyte membrane layer is between about 60 and 120micrometers thick. In some embodiments, a thin polymer electrolytemembrane layer is used, being between about 20 and 60 micrometers thick.In some embodiments, a relatively thick polymer electrolyte layer isused, between about 120 and 200 micrometers thick.

In some embodiments, a thinner cathode buffer layer is used with athinner polymer electrolyte membrane. This can facilitate movement ofthe CO₂ formed at the interface back to cathode, rather than to theanode. In some embodiments, a thicker cathode buffer layer is used witha thicker polymer electrolyte membrane. This can result in reducing cellvoltage in some embodiments.

Factors that can influence the thickness of a cathode buffer layerinclude the ion selectivity of the anion-conducting polymer, theporosity of the anion-conducting polymer, the conformality of theanion-conducting polymer coating the polymer electrolyte membrane.

Many anion-conducting polymers are in the range of 95% selective foranions, with about 5% of the current being cations. Higher selectivityanion-conducting polymers, with greater than 99% selectivity for anionscan allow for a reduction in a significant reduction in thickness whileproviding a sufficient buffer.

Mechanical strength of an anion-conducting layer can also influence itsthickness, with mechanical stable layers enabling thinner layers.Reducing porosity of an anion-conducting polymer may reduce thethickness of the anion-conducting layer.

In some implementations, a cathode buffer layer or otheranion-conducting polymer layer that abuts the polymer electrolytemembrane is between about 5 and 50 micrometers, 5 and 40 micrometers, 5and 30 micrometers, 10 and 25 micrometers, or 10 and 20 micrometersthick. Using a >99% selective polymer can allow the cathode buffer layerto be reduced to between 2 and 10 microns in some embodiments.

In some cases, the ratio of thicknesses of the polymer electrolytemembrane and the adjoining anion-conducting polymer layer is betweenabout 3:1-90:1 with the ratios at the higher end used with highlyselective anion-conducting polymer layers. In some embodiments, theratio is 2:1-13:1, 3:1-13.1, or 7:1-13.1.

In certain embodiments, a relatively thinner PEM improves some aspectsof the MEA's performance. Referring to FIG. 11 , for example, polymerelectrolyte membrane 1105 may have a thickness of about 50 micrometers,while the anion-conducting layer may have a thickness between about 10and 20 micrometers. A thin PEM favors movement of water generated at theAEM/PEM interface to move toward the anode. The pressure of gas on thecathode side of the cell can be 80-450 psi, which causes the water atthe interface to move to the anode. However, in some instances, a thickPEM can cause the majority of water to move through the AEM to thecathode, which leads to flooding. By using a thin PEM, flooding can beavoided.

In some embodiments, a thin PEM may have a thickness of 10 micrometersto 50 micrometers, 30 micrometers to 50 micrometers, or 25 micrometersto 35 micrometers. In some such embodiments, the AEM may have a similarthickness to the PEM, such as 5 micrometers to 50 micrometers, 5micrometers to 30 micrometers, or 10 micrometers to 20 micrometers. Theratio of PEM:AEM thicknesses could be 1:2 to 1:1 when PEMs withthicknesses of 10-30 micrometers are used, 1:2 to 2:1 when PEM thicknessis 30-50 micrometers, or 1:1 to 3:1 when PEM thickness is 20-35micrometers. As described further below, AEMs in these thickness rangesmay be useful for water management.

Commercially available anion exchange membranes and cation exchangemembranes typically have known thicknesses. For example, Nafion®membranes have the following dry thicknesses:

Membrane Type Thickness (μm) Nafion 117 183 Nafion 115 127 Nafion 21125.4

Such known thickness can be used to determine thickness ratios. Forexample, if an AEM has a thickness between approximately 200 nm and 100μm, between 300 nm and 75 μm, between 500 nm and 50 μm as describedabove in the discussion of the cathode buffer layer, the PEM:AEMthickness ratio can be determined as follows:

PEM Membrane Type Example ranges of PEM:AEM N117 1.83-915; 2.44-610;3.66-366 N115 1.27-635; 1.69-423; 2.54-254 N211 0.25-127; 0.34-84.7;0.51-50.8

An AEM may have a thickness that aids in water management, as discussedfurther below.

Water Management

As described above, one of the key challenges in a CO_(x) electrolyzeris managing water in the cathode due to the need to have water presentto hydrate the polymer-electrolyte and/or participate in the CO_(x)reduction reaction but not so much water that it blocks the transport ofCO_(x) to the cathode catalyst. Water can transport predominantly by twomethods in a polymer electrolyte system: by electro-osmotic drag and bydiffusion. Through diffusion, water will move from areas of highconcentrations to low concentrations, the rate of water transportdepends on the diffusion coefficient that is an inherent property of thepolymer electrolyte material. Electro-osmotic drag is the movement ofwater molecules with ions as they travel through thepolymer-electrolyte. Water in a cation exchange membrane system willtransport along with the movement of cations from the anode to thecathode, whereas water moves in the opposite direction with anions in ananion exchange membrane system.

With a bipolar membrane (including a cation exchange membrane and ananion exchange membrane), the net movement of water from the anode tothe cathode can be managed by changing the thickness of theanion-exchange and cation-exchange polymer electrolyte layers and/ortheir material properties.

In some embodiments, an AEM may have a thickness between 5 and 80micrometers, 5 and 50 micrometers, 5 and 40 micrometers, or 5 and 30micrometers. As described below, relatively thick AEMs can aid in watermanagement and in preventing delamination, which prolongs lifetime.However, the thickness also contributes to higher voltages and lowerefficiencies. Thus, in some embodiments, the AEM may be no more than 50microns thick.

The tables below show net water transported from the anode to thecathode of the CO_(x) electrolyzer per ionic charge moved through thepolymer-electrolyte when the thickness of the anion-exchange polymerelectrolyte layer and cation-exchange membrane thickness are varied.When the anion-exchange polymer-electrolyte layer thickness increases,the net movement of water from the anode to the cathode decreases.Increasing the molecular weight of the anion-exchange polymer, whichreduces the diffusion coefficient of water through the anion-exchangelayer has a similar effect of decreasing the net movement of water perionic charge from the anode to the cathode of the device.

Nafion 115 (PFSA cation exchange membrane thickness 127 microns)

Anion-exchange Anion-exchange Water moved from polymer electrolytepolymer-electrolyte anode to cathode of COx layer thickness (um) MW(kg/mol) electrolyzer per charge 14-15 33 3.07 17.5-18.5 33 2.80 20-2133 2.44

Nafion 212 (PFSA cation exchange membrane thickness 50.8 microns)

Anion-exchange Anion-exchange Water moved from polymer polymer- anode toelectrolyte layer electrolyte MW cathode of COx thickness (um) (kg/mol)electrolyzer per charge 22 33 1.25 8 33 2.16

Thus, in some embodiments, the ratio of the cation exchange membranethickness:anion exchange membrane thickness (i.e., the PEM:AEM ratio) ina bipolar MEA is no more than 7:1, 5:1, 3:1, 2:1, 1.5:1, 1:1, or 1:1.5.

Nafion 115 (PFSA cation exchange membrane thickness 127 microns) withanion-exchange polymer electrolyte layers with different molecularweights.

Anion-exchange Anion-exchange Water moved from polymer electrolytepolymer-electrolyte anode to cathode of COx layer thickness (um) MW(kg/mol) electrolyzer per charge 14-15 33 3.07 14-15 77 2.55 14-15 902.48

Thus, in some embodiments, the molecular weight of the anion exchangepolymer electrolyte may be at least 50 kg/mol, at least 60 kg/mol, atleast 70 kg/mol, at least 80 kg/mol, or at least 90 kg/mol.

In some embodiments, the AEM polymer may be crosslinked to decreasewater movement from the anode to the cathode.

FIG. 12 shows Faraday efficiency for COx electrolyzers having bipolarMEAs with different thicknesses of AEM. Nafion 115 (127 micron) was usedfor the PEM. Also shown are results for an MEA with no AEM. FIG. 13shows cell voltages.

The electrolyzers were ramped to a high current density of 300 mÅ/cm².Faraday efficiency is the efficiency with which charge is transferred ina system facilitating an electrochemical reaction. Notably, the 0microns (no AEM) has a Faraday efficiency near 0 and the MEA with the3.5 micron AEM has a Faraday efficiency below 80%. This indicates thatthere is a minimum thickness of AEM for good performance in a bipolarMEA, which in some embodiments, may be 5 microns, or 7 microns. Similarresults are expected for other operating conditions and bipolar MEAs.

MEAs with AEMs between 7.5 microns and 25 microns had near 100% Faradayefficiencies over the course of operation. The 30 micron AEM MEAoperated with near 100% Faraday efficiency at onset with performancedecreasing at around 6 hours. This indicates that there may bedelamination occurring. Modifications to the fabrication and/oroperating conditions may be made to reduce delamination and achieveperformance comparable to the 7.5, 15, and 25 micron AEM MEAs for MEAshaving AEMs up to and 50 microns.

CO_(x) Reduction Reactor (CRR)

FIG. 14 is a schematic drawing that shows the major components of aCO_(x) reduction reactor (CRR) 1405, according to an embodiment of theinvention.

The CRR 1405 has a membrane electrode assembly 1400 as described abovein reference to FIGS. 2A-2A. The membrane electrode assembly 1400 has acathode layer 1420 and an anode layer 1440, separated by an ionconducting polymer layer 1460. The ion conducting polymer layer 1460 caninclude 3 sublayers: a cathode buffer layer 1425, a polymer electrolytemembrane 1465, and an optional anode buffer layer 1445. As discussedabove, cathode buffer layer 1425 may be omitted in some embodiments. Inaddition, the CRR 1405 has a cathode support structure 1422 adjacent tothe cathode layer 1420 and an anode support structure 1442 adjacent tothe anode layer 1440.

In one embodiment, the cathode layer 1420 contains an ion-conductingpolymer as described in Class A above, the anode layer 1440 contains anion-conducting polymer as described in Class C above, and the polymerelectrolyte membrane 1465 contains an ion-conducting polymer asdescribed as Class C above. If present, in one arrangement, the cathodebuffer layer 1425 contains at least two ion-conducting polymers: one asdescribed in Class A and one as described in Class B in Table 4 above.In other arrangements, the cathode buffer layer 1425 may contain oneion-conducting polymer, e.g., one as described in Class A or one asdescribed in Class B.

In another embodiment, the cathode layer 1420 contains both anion-conducting polymer as described in Class A and an ion-conductingpolymer as described in Class B, the anode layer 1440 contains anion-conducting polymer as described in Class C, the polymer electrolytemembrane 1465 contains an ion-conducting polymer as described in ClassA, the cathode buffer layer 1425 contains both an ion-conducting polymeras described in Class A and an ion-conducting polymer as described inClass B, and the anode buffer layer 1445 contains an ion-conductingpolymer as described in Class C. Other combinations of ion-conductingpolymers as described above may be used.

In some embodiments, the cathode layer 1420 contains an ion-conductingpolymer as described in Class A and the anode layer 1440 contains anion-conducting polymer as described in Class C.

The cathode support structure 1422 has a cathode polar plate 1424, e.g.,made of graphite, to which a voltage can be applied. There can be flowfield channels, such as serpentine channels, cut into the insidesurfaces of the cathode polar plate 1424. There is also a cathode gasdiffusion layer 1426 adjacent to the inside surface of the cathode polarplate 1424. In some arrangements, there is more than one cathode gasdiffusion layer (not shown). The cathode gas diffusion layer 1426facilitates the flow of gas into and out of the membrane electrodeassembly 1400. An example of a cathode gas diffusion layer 1426 is acarbon paper that has a carbon microporous layer.

The anode support structure 1442 has an anode polar plate 1444, usuallymade of metal, to which a voltage can be applied. There can be flowfield channels, such as serpentine channels, cut into the insidesurfaces of the anode polar plate 1444. There is also an anode gasdiffusion layer 1446 adjacent to the inside surface of the anode polarplate 1444. In some arrangements, there is more than one anode gasdiffusion layer (not shown). The anode gas diffusion layer 1446facilitates the flow of gas into and out of the membrane electrodeassembly 1400. An example of an anode gas diffusion layer 1446 is atitanium mesh or titanium felt. In some arrangements, the gas diffusionlayers 1426, 1446 are microporous.

There are also inlets and outlets (not shown) associated with thesupport structures 1422, 1442, which allow flow of reactants andproducts, respectively, to the membrane electrode assembly 1400. Thereare also various gaskets (not shown) that prevent leakage of reactantsand products from the cell.

In one embodiment, a direct current (DC) voltage is applied to themembrane electrode assembly 1400 through the cathode polar plate 1424and the anode polar plate 1442. Water is supplied to the anode 1440 andis oxidized over an oxidation catalyst to form molecular oxygen (O₂),releasing protons (H+) and electrons (e−). The protons migrate throughthe ion conducting polymer layer 1460 toward the cathode layer 1420. Theelectrons flow through an external circuit (not shown). In oneembodiment of the invention, the reaction is described as follows:

2H₂O→4H⁺+4e ⁻+O₂

In other embodiments of the invention, other reactants can be suppliedto the anode layer 1440 and other reactions can occur. Some of these arelisted in a table above.

The flow of reactants, products, ions, and electrons through a CRR 1505is indicated in FIG. 15 , according to an embodiment.

The CRR 1505 has a membrane electrode assembly 1500 as described abovein reference to FIGS. 2 a-2 c . The membrane electrode assembly 1500 hasa cathode layer 1520 and an anode layer 1540, separated by an ionconducting polymer layer 1560. The ion conducting polymer layer 1460 caninclude three sublayers: a cathode buffer layer 1525, a polymerelectrolyte membrane 1565, and an optional anode buffer layer 1545. Asdiscussed above, cathode buffer layer 1525 may be omitted in someembodiments. In addition, the CRR 1505 has a cathode support structure1522 adjacent to the cathode layer 1520 and an anode support structure1542 adjacent to the anode layer 1540.

The cathode support structure 1522 has a cathode polar plate 1524, whichcan be made of graphite, to which a voltage can be applied. There can beflow field channels, such as serpentine channels, cut into the insidesurfaces of the cathode polar plate 1524. There is also a cathode gasdiffusion layer 1526 adjacent to the inside surface of the cathode polarplate 1524. In some arrangements, there is more than one cathode gasdiffusion layer (not shown). The cathode gas diffusion layer 1526facilitates the flow of gas into and out of the membrane electrodeassembly 1500. An example of a cathode gas diffusion layer 1526 is acarbon paper that has a carbon microporous layer.

The anode support structure 1542 has an anode polar plate 1544, usuallymade of metal, to which a voltage can be applied. There can be flowfield channels, such as serpentine channels, cut into the insidesurfaces of the anode polar plate 1544. There is also an anode gasdiffusion layer 1546 adjacent to the inside surface of the anode polarplate 1544. In some arrangements, there is more than one anode gasdiffusion layer (not shown). The anode gas diffusion layer 1546facilitates the flow of gas into and out of the membrane electrodeassembly 1500. An example of an anode gas diffusion layer 1546 is atitanium mesh or titanium felt. In some arrangements, the gas diffusionlayers 1526, 1546 are microporous.

There can also be inlets and outlets associated with the supportstructures 1522, 1542, which allow flow of reactants and products,respectively, to the membrane electrode assembly 1500. There can also bevarious gaskets that prevent leakage of reactants and products from thecell.

CO_(x) can be supplied to the cathode 1520 and reduced over CO_(x)reduction catalysts in the presence of protons and electrons. The CO_(x)can be supplied to the cathode 1520 at pressures between 0 psig and 1000psig or any other suitable range. The CO_(x) can be supplied to thecathode 1520 in concentrations below 100% or any other suitablepercentage along with a mixture of other gases. In some arrangements,the concentration of CO_(x) can be as low as approximately 0.5%, as lowas 5%, or as low as 20% or any other suitable percentage.

In one embodiment, between approximately 10% and 100% of unreactedCO_(x) is collected at an outlet adjacent to the cathode 1520, separatedfrom reduction reaction products, and then recycled back to an inletadjacent to the cathode 1520. In one embodiment, the oxidation productsat the anode 1540 are compressed to pressures between 0 psig and 1500psig.

In one embodiment, multiple CRRs (such as the one shown in FIG. 14 ) arearranged in an electrochemical stack and are operated together. The CRRsthat make up the individual electrochemical cells of the stack can beconnected electrically in series or in parallel. Reactants are suppliedto individual CRRs and reaction products are then collected.

The major inputs and outputs to the reactor are shown in FIG. 16 .CO_(x), anode feed material, and electricity are fed to the reactor.CO_(x) reduction product and any unreacted CO_(x) leave the reactor.Unreacted CO_(x) can be separated from the reduction product andrecycled back to the input side of the reactor. Anode oxidation productand any unreacted anode feed material leave the reactor in a separatestream. Unreacted anode feed material can be recycled back to the inputside of the reactor.

Various catalysts in the cathode of a CRR cause different products ormixtures of products to form from CO_(x) reduction reactions. Examplesof possible CO_(x) reduction reactions at the cathode are described asfollows:

CO₂+2H⁺+2e→CO+H₂O

2CO₂+12H⁺⁺12e→CH₂CH₂+4H₂O

2CO₂+12H⁺+12e→CH₃CH₂OH+3H₂O

CO₂+8H⁺+8e→CH₄+2H₂O

2CO+8H⁺+8e→CH₂CH₂+2H₂O

2CO+8H⁺+8e→CH₃CH₂OH+H₂O

CO+6H⁺+8e→CH₄+H₂O

In another embodiment of the invention, a method of operating a COxreduction reactor, as described in the embodiments of the inventionabove, is provided. It involves applying a DC voltage to the cathodepolar plate and the anode polar plate, supplying oxidation reactants tothe anode and allowing oxidation reactions to occur, supplying reductionreactants to the cathode and allowing reduction reactions to occur,collecting oxidation reaction products from the anode; and collectingreduction reaction products from the cathode.

In one arrangement, the DC voltage is greater than −1.2V. In variousarrangements, the oxidation reactants can be any of hydrogen, methane,ammonia, water, or combinations thereof, and/or any other suitableoxidation reactants. In one arrangement, the oxidation reactant iswater. In various arrangements, the reduction reactants can be any ofcarbon dioxide, carbon monoxide, and combinations thereof, and/or anyother suitable reduction reactants. In one arrangement, the reductionreactant is carbon dioxide.

In another specific example, the COx reduction reactor includes amembrane electrode assembly, which includes a cathode layer thatincludes a reduction catalyst and a first anion-conducting polymer(e.g., FumaSep FAA-3, Sustainion, Tokuyama anion exchange polymer). Thereactor also includes an anode layer that includes an oxidation catalystand a first cation-conducting polymer (e.g., Nafion 324, Nafion 350,Nafion 417, Nafion 424, Nafion 438, Nafion 450, Nafion 521, Nafion 551,other Nafion formulations, Aquivion, GORE-SELECT, Flemion, PSFA, etc.).The reactor also includes a membrane layer that includes a secondcation-conducting polymer, wherein the membrane layer is arrangedbetween the cathode layer and the anode layer and conductively connectsthe cathode layer and the anode layer. The reactor also includes acathode manifold coupled to the cathode layer, and an anode manifoldcoupled to the anode layer. In this example, the cathode manifold caninclude a cathode support structure adjacent to the cathode layer,wherein the cathode support structure includes a cathode polar plate, acathode gas diffusion layer arranged between the cathode polar plate andthe cathode layer, a first inlet fluidly connected to the cathode gasdiffusion layer, and a first outlet fluidly connected to the cathode gasdiffusion layer. Also in this example, the anode manifold can include ananode support structure adjacent to the anode layer, wherein the anodesupport structure includes an anode polar plate, an anode gas diffusionlayer arranged between the anode polar plate and the anode layer, asecond inlet fluidly connected to the anode gas diffusion layer, and asecond outlet fluidly connected to the anode gas diffusion layer. In arelated example, the membrane electrode assembly of the reactor includesa cathode buffer layer that includes a second anion-conducting polymer(e.g., FumaSep FAA-3, Sustainion, Tokuyama anion exchange polymer), andis arranged between the cathode layer and the membrane layer andconductively connects the cathode layer and the membrane layer. Thebuffer layer(s) of this example (e.g., cathode buffer layer, anodecathode layer) can have a porosity between about 1 and 90 percent byvolume, but can alternatively have any suitable porosity (including,e.g., no porosity). However, in other arrangements and examples, thebuffer layer(s) can have any suitable porosity (e.g., between 0.01-95%,0.1-95%, 0.01-75%, 1-95%, 1-90%, etc.). In a related example, the firstand second anion-conducting polymers of the membrane electrode assemblycan be the same anion-conducting polymer (e.g., comprised of identicalpolymer formulations).

In the description above, the terms “micrometers” and “microns” and theabbreviations “μm” and “um” are used interchangeably to mean microns.Unless otherwise noted, ranges in this document (e.g., 10 micrometers to20 micrometers, 0.25-127, between 1 and 90%, etc.) include the endpointsof those ranges.

This invention has been described herein in considerable detail toprovide those skilled in the art with information relevant to apply thenovel principles and to construct and use such specialized components asare required. However, it is to be understood that the invention can becarried out by different equipment, materials and devices, and thatvarious modifications, both as to the equipment and operatingprocedures, can be accomplished without departing from the scope of theinvention itself.

1. A membrane electrode assembly comprising: a cathode catalyst layer;an anode catalyst layer; and a bipolar membrane disposed between thecathode catalyst layer and the anode catalyst layer, wherein the bipolarmembrane comprises an anion-conducting polymer layer, acation-conducting polymer layer, and a bipolar interface between theanion-conducting polymer layer and the cation-conducting polymer layer,wherein the cation-conducting polymer layer is disposed between theanode catalyst layer and the anion-conducting polymer layer, and whereinthe bipolar interface is characterized by or comprises one or more of:covalent cross-linking of the cation-conducting polymer layer with theanion-conducting polymer layer; interpenetration of the anion-conductingpolymer layer and the cation-conducting polymer layer; and a layer of asecond anion-conducting polymer, wherein the ion exchange capacity ofthe second anion-conducting polymer is higher than the ion exchangecapacity of the anion-conducting polymer of the anion-conducting polymerlayer.
 2. The membrane electrode assembly of claim 1, wherein thebipolar interface is characterized by interpenetration of theanion-conducting polymer layer and the cation-conducting polymer layerand wherein the bipolar interface comprises protrusions having adimension of between 10 μm-1 mm in a plane parallel to theanion-conducting polymer layer (the in-plane dimension).
 3. The membraneelectrode assembly of claim 1, wherein the bipolar interface ischaracterized by interpenetration of the anion-conducting polymer layerand the cation-conducting polymer layer and wherein the bipolarinterface comprises protrusions each having a thickness of between 10%to 75% of the total thickness of the anion-conducting polymer layer. 4.The membrane electrode assembly of claim 1, wherein the bipolarinterface is characterized by interpenetration of the anion-conductingpolymer layer and the cation-conducting polymer layer and wherein thebipolar interface comprises a gradient of the anion-conducting polymerand/or the cation-conducting polymer.
 5. The membrane electrode assemblyof claim 1, wherein the bipolar interface is characterized byinterpenetration of the anion-conducting polymer layer and thecation-conducting polymer layer and wherein the bipolar interfacecomprises a mixture of the anion-conducting polymer and/or thecation-conducting polymer.
 6. The membrane electrode assembly of claim1, wherein the bipolar interface comprises a layer of a secondanion-conducting polymer, and further wherein the thickness of the layerof the second anion-conducting polymer is between 0.1% and 10% of thethickness of the anion-conducting polymer layer.
 7. The membraneelectrode assembly of claim 1, wherein the bipolar interface comprises alayer of a second anion-conducting polymer and further wherein thesecond anion-conducting polymer has an ion exchange capacity (IEC) ofbetween 2.5 and 3.0 mmol/g.
 8. The membrane electrode assembly of claim7, wherein the anion-conducting polymer has an IEC of between 1.5 and2.5 mmol/g.
 9. The membrane electrode assembly of claim 1, wherein thebipolar interface comprises a layer of a second anion-conducting polymerand wherein the second anion-conducting polymer has a lower water uptakethan that of the anion-conducting polymer of the anion-conductingpolymer layer.
 10. The membrane electrode assembly of claim 1, whereinthe bipolar interface comprises covalent crosslinking of thecation-conducting polymer layer and anion-conducting polymer layer andwherein the covalent crosslinking comprises a material comprising astructure of one of formulas (I)-(V):

or a salt thereof, wherein: each of R⁷, R⁸, R⁹, and R¹⁰ is,independently, an electron-withdrawing moiety, H, optionally substitutedaliphatic, alkyl, heteroaliphatic, heteroalkylene, aromatic, aryl, orarylalkylene, wherein at least one of R⁷ or R⁸ can include theelectron-withdrawing moiety or wherein a combination of R⁷ and R⁸ or R⁹and R¹⁰ can be taken together to form an optionally substituted cyclicgroup; Ar comprises or is an optionally substituted aromatic or arylene;each of n is, independently, an integer of 1 or more; each of rings a-ccan be optionally substituted; and rings a-c, R⁷, R⁸, R⁹, and R¹⁰ canoptionally comprise an ionizable or ionic moiety.
 11. The membraneelectrode assembly of claim 10, wherein R⁷ or R⁸ comprises theelectron-withdrawing moiety selected from the group consisting of anoptionally substituted haloalkyl, cyano, phosphate, sulfate, sulfonicacid, sulfonyl, difluoroboranyl, borono, thiocyanato, and piperidinium.12. The membrane electrode assembly of claim 1, wherein the bipolarinterface comprises covalent crosslinking of the cation-conductingpolymer layer and anion-conducting polymer layer and wherein thecovalent crosslinking comprises a material comprising a structure of oneof the following formulas:

or a salt thereof, wherein: Ar is or comprises an optionally substitutedarylene or aromatic; Ak is or comprises an optionally substitutedalkylene, haloalkylene, aliphatic, heteroalkylene, or heteroaliphatic;and L is a linking moiety, and wherein one or Ar, Ak, and/or L isoptionally substituted with one or more ionizable or ionic moieties. 13.The membrane electrode assembly of claim 1, wherein the bipolarinterface comprises covalent crosslinking of the cation-conductingpolymer layer and anion-conducting polymer layer and wherein thecovalent crosslinking comprises a crosslinker comprising a structure ofone of the following formulas:

wherein: Ak is an optionally substituted aliphatic or an optionallysubstituted alkylene; Ar is an optionally substituted aromatic or anoptionally substituted arylene; L is a linking moiety; L3 is an integerthat is 2 or more; and X′ is absent, —O—, —NR^(N1)—, —C(O)—, or -Ak-, inwhich R^(N1) is H or optionally substituted alkyl, and Ak is optionallysubstituted alkylene, optionally substituted heteroalkylene, optionallysubstituted aliphatic, or optionally substituted heteroaliphatic. 14.The membrane electrode assembly of claim 10, wherein the covalentcrosslinking comprises a material comprising one or more ionizable orionic moieties selected from the group consisting of -L^(A)-X^(A),-L^(A)-(L^(A′)-X^(A))_(L2), -L^(A)-(X^(A)-L^(A′)-X^(A′))_(L2), and-L^(A)-X^(A)-L^(A′)-X^(A′)-L^(A″)-X^(A″); wherein: each L^(A), L^(A′),and L^(A″) is, independently, a linking moiety; each X^(A), X^(A′), andX^(A″) comprises, independently, an acidic moiety, a basic moiety, amulti-ionic moiety, a cationic moiety, or an anionic moiety; and L2 isan integer of 1 or more.
 15. The membrane electrode assembly of claim14, wherein each X^(A), X^(A′), and X^(A″) comprises, independently,carboxy, carboxylate anion, guanidinium cation, sulfo, sulfonate anion,sulfonium cation, sulfate, sulfate anion, phosphono, phosphonate anion,phosphate, phosphate anion, phosphonium cation, phosphazenium cation,amino, ammonium cation, heterocyclic cation, or a salt form thereof. 16.The membrane electrode assembly of claim 12, wherein the linking moietycomprises a covalent bond, spirocyclic bond, —O—, —NR^(N1)—, —C(O)—,—C(O)O—, —OC(O)—, —SO₂—, optionally substituted aliphatic, alkylene,alkyleneoxy, haloalkylene, hydroxyalkylene, heteroaliphatic,heteroalkylene, aromatic, arylene, aryleneoxy, heteroaromatic,heterocycle, or heterocyclyldiyl.
 17. A membrane electrode assemblycomprising: a cathode layer; an anode layer; and a bipolar membranedisposed between the cathode layer and the anode layer, wherein thebipolar membrane comprises a cation-conducting polymer layer and ananion-conducting polymer layer, wherein the cation-conducting polymerlayer is disposed between the anode layer and the anion-conductingpolymer layer, and wherein the thickness of the anion-conducting polymerlayer is between 5 and 80 micrometers.
 18. The membrane electrodeassembly of claim 17, wherein the thickness of the anion-conductingpolymer layer is between 5 and 50 micrometers.
 19. The membraneelectrode assembly of claim 17, wherein the thickness of theanion-conducting polymer layer is between 5 and 40 micrometers.
 20. Themembrane electrode assembly of claim 17, wherein the thickness of theanion-conducting polymer layer is between 5 and 30 micrometers.